^^ what he said. SRT is text and timestamps, SUP is images and timestamps, essentially.
To be honest, SRT is probably truly fine for almost everyone. But, especially with other languages, having your player so much as select the wrong subtitling font can be a very serious problem. For non-English subtitles, the player-positioned SRT subs might overlap the Greedo subs, and so on. SUP subtitles look how I designed them to look (they look exactly like the screencaps in Project Threepio’s first post). But SRT may be close enough for a lot less effort, so I can’t blame people for using them (who knows what they look like, but they’re the right words at the right times, so there you go).
And also, I’m the first to admit my “matching” subs are a total gimmick, but that whole concept is pretty much impossible with SRT.