Hi, Stewart here from the Firebase team.

https://firebase.google.com/docs/admob/admob-firebase has a pretty good explaination of the benefits. For example, using AdMob + Firebase you'll immediately be able to take advantage of Firebase Analytics to "measure sessions, user demographics, revenue from in-app products" and you'll have a relatively easy path to start utilize other Firebase features - if you want - in future.

Cheers, Stewart