We have Chinese subtitles in Taiwan too, and pretty much everyone here understands Mandarin Chinese. Personally, I always skim the subtitles even though I can understand fine just by listening; it's just a lifelong habit. I know my friends and family does the same because this is something that we've talked about: why other countries don't have subtitles. It doesn't have anything to do with fluency; sometimes it's just hard to understand what people are saying 100% even if they're talking in your native tongue. They could be talking too low, talking too fast, have a weird accent, or your hearing could just be off sometimes. When I watch movies in US theaters sometimes I can't understand what the characters are saying; and I know it's not just me because when I ask my friends they didn't understand either. For a more practical reason, when I turn the volume down for whatever reason I can still understand by reading the subtitles. Also, for children I think it improves their reading skills. My mom said my sister learned to read a little before even starting elementary school just by watching TV with subtitles.

the subtitle is for people who can not understand putonghua very well. and the subtitle can also clear the ambiguity in case some words have the same pronounciation.

CCTV or film subtitles have two purposes, one is that China has a lot of different dialects, even if not speak Mandarin, but they know the Chinese characters, so they can communicate with the subtitles. Second, in order to the deaf, they can not hear, so with the subtitles can understand the meaning. 希望能够帮到您~

And I think it's great!

This question has been asked times before. It's interesting and amazing to foeigners, probably. In my personal experience or memory, the subtitles firstly appeared only in TV news where people spoke foreign languages, ethnic languages or Chinese dialects other than mandanrin. Later, for reason I do not know, subtitles began to appeare in more and more TV programs. I think it's mainly for people who have hearing disorder. I do not think it's for foreigners to learn Chinese, but practically it does help with that. :)