[keycloak-user] Stateless REST webservice registration

I'm trying to expand my knowledge about Keycloak and OpenID Connect. Is it necessary for a stateless REST webservice to be registered as a client in Keycloak? The token send to the REST service is signed, so the REST service could verify the authenticity and validity of the token if it has the public key of the keycloak server. Why would there be any need for direct communication between the REST service and Keycloak?