When subs are in one's native language:
When your skill in the language you are learning is low, then subtitles are useful to understand the scenario and to some extent the dialog. But I think it is a problem to read one language while listening to another (anyway I can't).
What I do is watch the movie without subtitles first, and try to understand as much as I can. Then go back and watch it with the subtitles, more to better understand what is happening and being said, to help fill in what I missed the first time.
Then watch it the third time (and times after that). Have the subtitles on screen, but don't read them unless you get to something you don't understand. Then you can pause and translate as needed.
And gradually wean yourself off of the subs.
When subs are in the learning language, I think the only benefit would be reading comprehension in that language (text vs. spoken). Speech patterns/accent etc. can make the spoken dialog difficult to make out. If that is the case, then same language subs can be useful. But if you come across vocabulary or grammar you don't understand, it won't really help you will still have to look it up to further your learning.