Japaneses put captions on everything due to the large amount of homophones in the Japanese speaking language; Words sound the same but are written with different kanji, having the text form avoid confusion.
They handle it just fine, of course. I disagree with the statement that Japanese has a relatively high incidence of homophones. It is true that Japanese TV shows, particularly variety shoes and lighter fare like that, have a lot of on-screen captions and I don’t know the reason for that, but I doubt it’s homophones, and at any rates more “serious” shows like dramas as well as all movies are caption-free (but will likely have a subtle track you can enable).
I lived in Japan in the 90s and watched a LOT of TV there. From my experience most subtitling on TV fell into 2 categories:
On anime and children's shows, the theme songs were frequently subtitled. I assumed this was so viewers could appreciate the lyrics
Comedy, skit and variety shows, where the dialog and commentary is mostly banter amongst a cast of "wacky" hosts. Here, subtitles (almost always in a garish, colorful font) served to punctuate jokes or funny lines, in the same way that a laugh track on an American sitcom is used to let the audience know when to laugh (even though the Japanese shows usually had a laughing studio audience as well).
Some shows did (do?) go overboard with the "comedic" subtitles, to a point where they were subtitling almost every other line a host said.
Going by my poor Japanese listening skills and some interviews from documentaries I've seen, Japanese speakers mostly stick to common words and idioms in conversations. They also "over-explain" by repeating, rephrasing, or even reacting to their own points.
So, it's just like conversations in any language: basic, rambling, and emotive.