A free account is an easy way to test out the platform, give it a test ride, see if it works for you. If it does, you can pay for a Pro (or Team) subscription, and you get to use your own domain, and keep all the posts you already made on the free account, with all their comments and replies and whatnots.

Or, if it works out, and you want to self host, you can do that, too!

The difference between write.as and medium and other enshittified things, though, is that write.as is not VC funded, and Matt has no interest in making an "exit". Even if there are things I disagree on with him (eg, CLAs), I trust Matt to not enshittify write.as anytime soon. He's been running things for almost a decade now, remarkably well.