What service is required to log onto a domain but is not required to log onto a workgroup?

The answer is "Netlogon server".

Explanation:

It is a process on Microsoft windows which verifies clients and other domain services. normal operating in the background, since it is a service and its not an application, only if it is disabled manually or from an error in runtime.

It can interrupt or restart from both the command-line. It needed to be functioning to allow a secure channel to be formed between both the server and the DC, this also enables Dynamic DNS changes to operate.