App Engine is great when you want a scalable service that you use to talk directly to your backend services. Cloud Functions lets you extend an existing service

App Engine is great when you want a scalable service that you use to talk directly to your backend services. Cloud Functions lets you extend an existing service with custom code. The best example is Cloud Storage and thumbnailing. If you wanted to automatically create thumbnails for images you could do that easily with an endpoint on app engine. The only problem is you'd lose out on all the other features that Cloud Storage for Firebase offers like automatic resume and 3p authorization. Instead you could write a Cloud Function that creates thumbnails as a side effect and your client app keeps talking directly to a BaaS.