I subscribe to communities that I’ve a specific interest in but mostly browse by all. There are times I might want to post to a specific place so being subscribed makes that easier, and as Lemmy fills up I think I’m more likely to start using subscribed instead of all. Right now new communities are being created frequently and there’s also not enough posts happening on my subscribed communities to make it worth while yet, but it will soon enough.

I think subscriptions works best when you keep it small. Like you say, if you subscribe to everything then it’s no different to all, except you miss some potentially interesting new communities.