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.