This module is simply because we're lazy. Sometimes you have to update not just a few modules, but a LOT of modules. While "drush up" will give you everything, you can't just update a selection of modules unless you want to type them all in manually, and that's no fun.
Drush Update gives you a list of modules that have pending updates for you to check off, and then gives you the command to copy/paste into the console. Pretty simple, right?