I think they're mostly just there for emphasis and comedic effect! You'll notice that there are always subtitles for the punchlines of jokes and stuff. I really like them, I've gotten used to them and now I think they add a lot to the shows, especially variety shows!
As for news programs and stuff, I think they're just used as a way to summarize what a person is saying, since often they don't include every word.
And like you said, subtitles are also used when a person speaks a non-standard dialect of Japanese, but most of the time they aren't there because the person is hard to understand or anything
