The idea (maybe not very well communicated in the README and other docs[1]) is that each instance would correspond to a "subreddit" with its distinct community. The federated aspect however still allows users from other instances to subscribe/participate. In my personal opinion building services that group multiple interests is a step in the wrong direction creating redundancy to the concept of federation itself.

[1] https://man.sr.ht/~mariusor/go-activitypub/brutalinks/as-a-r...