It’s useful.
Let’s say you see someone who posts stuff you’re interested in. In a brief moment of absolute brilliance, you think to yourself “aha! Maybe this person follows other people whose content I would be interested in!”
So you check, and sure enough, there’s a bunch of interesting people listed. So you follow them as well. Your social graph grows, you have a better time there, the people you follow get better reach and gets to enjoy pleasant interactions with you. Everybody’s happy.
These social media platforms are designed to be public. If you want to do stuff in secret, do it somewhere else.
You cannot interact with microblog folks on Lemmy, unless they actively post something in a Lemmy community by tagging it. So if you want to combine microblogging with threaded discussions Mbin is the only platform that does both. Mbin lists followers publicly.
I think there are Mastodon forks (or configurations) that hide followers from the public though. But it will only ever be half hidden.