Discoverability, basically. You can get a lot of incredibly good information from Twitter with a very carefully curated follow list. You can get that from Mastodon as well, but you're going to have a much harder time finding the same people in a fully decentralized world. I think I'd rather that existence, which I'm noticing as I wind up in more niche Facebook groups and Discord servers than on broader platforms like Instagram or Twitter, but it's not zero value per se.