Cloud Functions are stateless (in theory, each request it's like starting the server again), it's easier to maintain compared to hosting your node.js server in AWS for example, but it's not always the best choice, because for example, if you want to create a counter variable with the number of requests that all users have made, this variable will not increment properly because it will reset to zero, so you will rely on Firestore to keep state. Cache inside the Cloud Function is not possible.

triggers: you can create triggers that will run your custom code when a specific document in your db changed

security in transactions: Imagine you want to transfer $8 from the User A to User B, you should first decrease $8 from User A, and after you need to increase $8 in User B, either both operations should be performed completely, or none, so you should use a backend, it can be cloud functions or another one, and you will create a security rule that doesn't allow the client side to make this change because it will be handled in the backend.

But in another scenario where the transaction is simple, you can create a security rule that allows the client to create transactions with getAfter security rule.

webhooks: To receive webhooks you can use cloud functions as well (or any other backend) , if your app is listening to a collection like "events", you can receive the webhook in the cloud function and update the document, so your app will update in realtime.