Others have pretty well covered how multiple communities serve different purposes. I’m going to cover why that isn’t a problem.

When you first decide to follow a community on Lemmy- either as a brand new user or an experienced Lemming branching into a new subject, there’s going to be a few ways you find a community.

The most obvious is a search, which will list the various communities. You’ll check the few most active, and either subscribe or skip each one. Multiple communities isn’t a concern because every Lemmy client will aggregate them in your feed. When you decide to start posting, you’ll have a decent feel for which one(s) are a good match for your content. The downside is cross posting, which is still a problem that needs to be solved.

The next option, mostly used by new users, is a direct link. Someone tells you about a specific community. You follow the link, read the posts, and decide to subscribe. Other, similar communities don’t matter- the person sending you the link already filtered it down for you.