Domain administrators are automatically granted Administrator access to all machines in the domain, without having to manage it at each server.
Domain Administrators have local-admin equivalent access on each machine, but local Administrators don't have admin access to domain-level administrator resources.
That's the main point and the difference -- centralized management of the domain credentials vs. individual credentials management at each server, and access scope.
Say you wanted to change the admin's password and you had 100 servers, would you rather update a single user's credentials in one place, or visit 100 servers to update the credentials, one at a time?
As well, Domain-level credentials get passed around securely from domain resource to domain resource as you access them, avoiding the need to log in to and/or store credentials for each device.
Say you created a new intranet site or web app for the admin, which requires Windows-level authentication. Would you rather add and maintain a single domain user's credential to the allow list, or add one for each possible machine-level administrator account?