securing APIs: JWT as OAuth2 bearer token vs. JWT directly

Question

Google APIs have a mechanism for server-to-server authentication. I believe this is the OAuth2 JWT bearer flow.

In this model, the API consumer uses a JWT to get an access token, and then uses the access token to make API calls.

I've also read about using JWT as OAuth2 access tokens.

What I'd like to understand is, why couldn't you just use a JWT to access Google APIs directly? What extra value does the access token provide?

Apologize if this is a naive question.