The Starter plan doesn’t come with any free domain name, only the Super Premium and higher plans include domains.
The “Free Domain!” is actually just a free subdomain. There is probably some utility to it, but if you’re paying for hosting, you should really have your own domain name too.
The higher plans do include a “free domain” by default, but you can also choose to get the plan without “free domain”, which is roughly $1 per month cheaper.