Just getting started with Vonk server.
I'm curious to know, under what circumstances do we need to persist FHIR resources in FHIR Server's storage?
You persist the resources in a storage when you want to be able to retrieve and/or use the data later.
It is up to you whether you do that with Vonk as a generic server - i.e. Vonk with its own storage, or with Vonk as a Facade to your back-end storage system. That greatly depends on use case, access to and availability of back-end systems, and design of the architecture, etc.