You buy a Tailscale contract for the same reason you buy a Red Hat contract. If something goes wrong, and you need to fix it fast, their experts will work on that, not yourself.

Also, Tailscale offers OAuth through large corporate providers; I'm not sure Headscale is going to support that. (Actually, this is why I don't use Tailscale for my private network: I don't want to depend on an external OAuth provider.)