I think they're useful when used to tag messages of a relevant topic, but not when they're overdone.

I don't particularly like seeing posts that are suffixed with 30 hashtags, personally. But having some relevant ones based on content can sometimes be useful for discovery.

I've followed a lot of accounts that use hashtags which have helped me discover them. And I've muted a lot of accounts that overdo it.

TL;DR balance is key, I guess. 🤷