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?
In short: The Fediverse is one universe and everyone is supposed to be able to interact with everyone. If you like the technology community on A better, use that. If you like the one on B more, you can engage there. It’s supposed to not matter a lot where exactly you are. But sure, you’re allowed to create yet another community about the same topic on C. Downside is, someone needs to upload content there and comment on things.