Some folks won't be able to access some communities due to defederation, and in other cases folks may want to start a community on their home instance for certain reasons. This isn't the end of the world; probably in the long run either everybody will centralize on the biggest one, or you'll end up with one or two alternatives that have a bit different culture or focus.

That being said, if you like to read comments, it can get confusing when a bunch of different communities all have high interaction posts about the same topic at the same time.