When is SeTcbPrivilege used? ("Act as part of the operating system.)

Acting as a part of the operating system allows you to do things like create login tokens. It's unlikely that you would ever need to write a service that uses this privilege unless you're writing an authentication provider.

Since you can create access tokens, you can act as any user. Of course, this means that you can run programs under the SYSTEM account, but there are much easier ways to run something as SYSTEM.