IAP controls access to your App Engine apps and Compute Engine VMs running on Google Cloud. It leverages user identity and the context of a request to determine if a user should be allowed access. IAP is a building block toward BeyondCorp, an enterprise security model that enables employees to work from untrusted networks without using a VPN.
By default, IAP uses Google identities and IAM. By leveraging Identity Platform instead, you can authenticate users with a wide range of external identity providers, such as:
Email/password
OAuth (Google, Facebook, Twitter, GitHub, Microsoft, etc.)
SAML
OIDC
Phone number
Custom
Anonymous
This is useful if your application is already using an external authentication system, and migrating your users to Google accounts is impractical.
https://cloud.google.com/iap/docs/using-tcp-forwarding#grant-permission

If your apps are fault-tolerant and can withstand possible instance preemptions, then preemptible instances can reduce your Compute Engine costs significantly. For example, batch processing jobs can run on preemptible instances. If some of those instances stop during processing, the job slows but does not completely stop.
Preemptible instances complete your batch processing tasks without placing additional workload on your existing instances and without requiring you to pay full price for additional normal instances.
https://cloud.google.com/compute/docs/instances/preemptible