A great use case would be subreddits where official authorities chime in on occasion. A real world example: on /r/Overwatch, a subreddit I manage, we have Blizzard (makers of the game) respond to posts on rare occasion. We want these threads to stand out, so we have some special CSS and actual flair text for these posts. As it exists, we have duplicate AutoModerator rules for all of our existing flairs, so that if a Blizzard employee comments on a post, we replace the current flair with a Blizzard flair, and replace the flair text with "{Category} | Blizzard Response".

Rather than having to do this for each and every flair template:

# Blizzard Response to eSports
moderators_exempt: false
author: 
  flair_css_class (includes): "blizz"
priority: -100
type: comment
parent_submission:
  flair_css_class: "b"
  set_flair: ["eSports | Blizzard Response", "b"]
  overwrite_flair: true

We might instead just do this, once:

# Blizzard Response
moderators_exempt: false
author: 
  flair_css_class (includes): "blizz"
priority: -100
type: comment
parent_submission:
  append_flair: ["Blizzard Response", "blizzresponse"] //append instead of set

It would be useful to be able to append text and append a CSS class (independently, ideally).