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.

Because the idea is to be one giant town hall/message board and to grow a social network based on who you follow.

If you don't want people to see what porn stars you follow? Make a second account to get horny on.

they wanted at least some feature parity with twitter in order to draw users expecting that environment.

I'm just from tumblr we don't do that over there

The Reddit-inspired instances like the Mbin and Lemmy-based ones may be of interest for you. The Lemmy ones, from what I can tell, always hide the follower list, and the Mbin ones allow the user to choose between showing and not showing. Also, both seem to be able to connect to Twitter-like instances, though UI for that part in the Mbin ones is pretty barebones and the Lemmy ones mix them up.

I find it generally weird that follows, likes, etc. are public. But I can't say I understand those type of platforms anyway.

Since I worked on a Mastodon client, I know that there is a hide_collections parameter in the account settings which allows you to not disclose publicly your followers and following.

It's in the PATCH v1/accounts/update_credentials API request body, see documentation here.