well I'd say since non-federated Lemmy is just a forum with a bunch of stuff for federation that you won't use, there's no point. if you want a forum, then Lemmy is the wrong answer. Lemmy is (or at least is designed to be) an open-source, federated copy of reddit, keeping the good parts while removing the corpo stuff and adding the benefits that open-source and federation bring. with only one instance, it's little more than a mediocre forum.