I remember arriving in Lisbon, leaving a favorable review for a restaurant because they were so nice to us, and Google sending me a notification that I'm now a local guide for Lisbon.

What exactly does that mean though? Is there any benefits to it? All I see is a badge/label, that's it?

There are some benefits that definitely used to exist, and maybe still exist, like early access to new features and additional Google Drive storage. But in practice today, the only real benefit is the badge.