I don't know if anybody else does this, but sometimes I go through a period where I'm less interested in a community (for instance, if it's fandom-related and the show is on hiatus) and at that point I'll unsubscribe from the community so I won't see posts on my reading page. If I unsubscribe and I'm not a member, then I'll likely forget what the community was called because it's no longer in the list of the communities I'm a member of. So being a member definitely has a benefit for me even if I can't post.