It is normally more where mods can dodge having to completely close down a thread/not have to delete a post as someone still might get their question answered, or the remaining info is OK but the rest, or leave a locked thread with the title there for all to see.
Anyway two main scenarios beyond simply quoting someone and only wanting to include the points you are addressing but want to make it clear to those reading the quote that you snipped a bit out.
1) It names a ROM site, torrent site, direct mentions a usenet group or direct mentions a subreddit or whatever where you can find either copyrighted material or material otherwise unsuitable for the forum.
2) You may also see it if keys are displayed. The rules surrounding keys are vague and not well tested in courts so while many normally far more conservative hackers (ones that would take great pains to avoid having their work used to make a ROM loader or something) might well post them then GBAtemp does not allow them. This is probably more of a hangover from the HDDVD key leaks but it is what it is and most people can still find keys if they need them.
