I’m sure there are some good reasons, but I don’t know what those reasons are.
I’ve noticed that sometimes the instance of the community doesn’t match the instance of the user who posted there, and I was wondering why they chose to post to that community instead of an equivalent one on the instance they joined. Are there pros and cons to doing this?
Yes, but they can’t make use of the name on Reddit. So if you didn’t like how r/movies was made, the odds are there’s no intuitive name for a subreddit to take left to try and compete with them.
Not that you even can effectively compete with them because subreddit promotion on reddit is total garbage.