What is the significance of the start-up sound of a system?

My understanding was that it came from a time when startup could take a while, during which the user's attention would be elsewhere and therefore worth notifying them when the computer was ready for use.

Something similar was mentioned in answer to another question:

The Operating System took so long to start up that the chime notified people that it was worth bothering to come back to the computer from, e.g., making a coffee. (Should we use a sound/jingle when users arrive on our site or open our app?)

According to comments made by Microsoft to Mashable, the startup sound was removed in Windows 10 because there's no need to draw attention to startup:

When we modernized the soundscape of Windows, we intentionally quieted the system. Using Windows 10, you will only hear sounds for things that matter to you. We removed the startup sound because startup is not an interesting event on a modern device. Picking up and using a device should be about you, not announcing the device's existence.

Part of it is iconic branding, much like putting the logo on the boot screen. I can still remember the startup sound from windows xp and playstation one.

To give auditory feedback to the user that the system has started loading.

Here are 3 reasons why:

1. Prevent users from hitting the power button multiple times.

   Users should not wonder whether the system is starting or not. If there was no starting sound how would users know that the system has successfully started? They might press the power button a couple of times not knowing that the system is already starting which can cause loss of information and a lot of headaches;

2. Display not working during repairs

   During hardware repairs when technicians haven't connected display or the display is not working and cannot get visual feedback. In this situation auditory feedback comes in very handy.

Update:

3. Cue for visually impaired (@Devin answer)

   Visually impaired people rely on hearing and touching when trying to understand the environment around them. If there are no auditory(sound) or haptic(touch/vibration) cues they won't know what is happening with the device.

Asides from old systems and their delay times at start, or branding considerations (both correct reasons), one reason that I might add is that it serves as a Sensory Cue.

This is true both for blind or impaired vision users, in which startup sound is of paramount importance (this sound is the only indication they have in order to know that the system is "ready to go") as well as "regular" users with no disabilities. In this regard, there are thousands of studies about sensory cues as start points that prepare the brain for a task.

Try to picture this (or even better, test if you have the means): simply think of yourself waiting for the system to start, then you hear the sound. Is your mood the same?

If you want, try doing this very simple experiment (we already did it!): simply call some people, maybe even your fellow co-workers or family, and film them from the point they press the ON button on a computer, until they hear the sound. 5-10 testers will be enough for this simple experiment. If your results are similar to ours, you'll see a very perceptible change on the testers. They will usually smile, or they will change their body position, or they will look more alert. Anyways, no matter the attitude they take, the important part is that most users (if not all) will react in some way

Note: New technologies will rely on aural(auditory)-haptic cues rather than only aural cues, so this answer is more related to desktop or laptop devices

Beeps during POST are there to help with troubleshooting. Think of it like debugging: if you can't tell why your script isn't working or where, it helps to have it echoing its status along the way in a very verbose manner so it becomes apparent where the issue is. It's like a ping.

POST does a lot of simple things very quickly to test itself (this is all in BIOS so it's actually the motherboard firmware testing itself, and associated components like processor and memory) like loading basic hardware drivers to talk with other components, and verifying its own hardware such as the speaker works with a quick beep.MIDI (Talking about the speaker on the motherboard itself here, not the auxiliary speakers you plug in to the back.)

So typically, the computer makes a "beep" only after it has tested and loaded the most basic-of-the-basics that make it possible for it to "beep". (i.e. indicates successful POST) Basically it is the response to your ping on the power button, "Hello?", "Yes, Hello! I have power and my basic hardware is functional."

So a post-POST beep is there to enhance the technician's experience, or the technical-user's.

Consider the example: You and I have just received the parts to build a new computer, so we unbox the motherboard to perform a benchtest to verify everything is functional first. We power on the system, and get no picture.

Here's a perfect example why those beeps are so important: if we get no beep, what could be wrong? 1) POST did not complete successfully, 2) Speaker hardware is damaged, 3) Major component missing (processor/memory not seated properly) 4) Manufacturer did not include BEEP after POST in firmware, so it still could have POSTed successfully and the issue really is the monitor, 5) cable, or 6) motherboard connector.

If we get one beep, that's easy: it's either the monitor or the cable (or the connector).

If we get more than one beep, that's also easy: obviously POST did not complete successfully, but now we can work out the issue (# of beeps = errorcode) quite easily with trivial research. Troubleshooting is fantastically difficult without verbosity.

After all this hardware stuff completes is where the OS takes over. While POST completes almost immediately, obviously OS takes a lot longer.

Hence, when BOOT has completed most OS's will play a "Welcome Sound" like the chimes on Windows XP or the Beatles' "A Day in the Life" ending used on the Mac.

This post-BOOT sound can be much higher quality and more detailed than the simple MIDI sounds of a motherboard speaker, so while this sound has been used for branding purposes as others have already mentioned, it's primary function is to indicate that it has finished booting. (As others' answers have already mentioned.)

Thus, the Welcome Sounds enhance the end-user's experience, for it indicates BOOT has completed and they can return from making coffee now.

Side Note: It depends on how old you are if you'll notice, that nowadays POST is obfuscated in favor of a manufacturer's logo, and it completes so quick on modern hardware you wouldn't see much anyway. The beep either happens so instantaneously (right after you press the power button) that you either don't notice it, or manufacturers have started disabling it by default. Both are options you can most probably turn back on in BIOS.

Edit: It's not even BIOS we use anymore, now it's called UEFI. I'm not even 30 yet and my old is showing.

Branding.

Of course there are other reasons to include it, like letting the user know the hardware is working, start up is finished and they can interact with the computer, etc. but it is an extremely powerful branding tool. It is familiar, it becomes expected, and if it's a good one, it can evoke strong feelings in us (see for instance http://gizmodo.com/end-of-an-era-goodbye-mac-startup-sound-1788383059 ). It reminds us of our relationship with the computer.

It is a notification that the software and hardware checks have passed (with no beep codes), and that the user is now able to access an operating system through a command interface shell or an associated GUI (Graphical User Interface) desktop.

Depends on what you mean by the startup, do you mean when you actually log in? Or do you mean when you turn the computer on, once you get to the login screen the noises made serve to do nothing at all and are just branding?

When you actually turn on the computer it can make a variety of noises most commonly heard are beeps, depending on the motherboard each beep be it single double triple etc.. or long drawn out "solid" pings so to speak are typically intended to alert the user to an issue such as a malfunction on the board its self or the failure of hardware; HDDs, GPUs, CPUs, unfortunately, more people are unaware of this than those that aren't which means these noises typically go ignored and if one happens to be an alert tone for a malfunction the issue only gets worse down the line.

At least for computers, beeps are a way to inform the user of problems if there is no (connection to the) screen.

Different mainboard brands have different beeps: Mainboard debug sounds so that a technician has a hint on where to start his search if the system is not booting.

There are different ways an OS uses to give feedback to the user aside from visual. In the case of desktop devices, sound is used to give feedback and communicate something happened or to call for the attention of the user.

As @MattObee states in his answer, a start-up sound tells the user that the system turned on and thus is responsive. Other sound notifications can be used when an error happened or some process finished. The same happens when the system is turned off.

In the case of Mobile devices the sound feedback can be substituted with haptic feedback.

If we include a Dial-up Internet connection as 'starting a system', then the dial up sounds are all actual line tones and are used to negotiate the exchange of information. The reason they are audible, as I understand it, is so that if the connection is not working an engineer (or someone with a manual) can figure out which stage of the connection is failing based on the tones and fix from there.