Like many said, it's not about profitability but sustainability. I signed up to donate $2 per month to help run the servers for lemmy.world. I'm very happy with this instance (and the fediverse in general) and want to contribute. There are plenty of other people willing to do the same. Together, we will make something much bigger and better than reddit over time.
I love their $8/month tier description: "The $8 verified user tier. You'll be allowed to place a blue checkmark behind your name. You'll have to do that yourself though. And you could also do that without donating ;-)."