Why using cloud functions to respond to database changes, when in Firestore there's data persistence?
This means each change in the database performed from the client, even when made offline, is saved and performed when the connection is back.
I don't get why using cloud functions to perform changes in database with firestore triggers, what's the benefit using them?
When you want exactly one source of logic to handle changes to documents (as opposed to all of your connected client apps trying to handle the change).
When you want to handle changes reliably and immediately while the client apps are not running at all.
When you want to run code that requires secure, privileged access to project resource that can't be trusted to run on user devices that are not under your direct control.
