After reading up on SNMP and some of the questions help here I think understand the agent role as a SNMP service to device (Like SQL, it is an API to storage).

When you execute a SQL query the SQL engine does all the work and returns the result - You don't need to be aware of how the storage and where the storage is done.

But MIBs are not actual storage , so what is the role of my agent?
if the agent only register the MIB like i follow in this tutorial, is it not used as handler at all?

Say that I want monitor my application's pending request queue, so I want an agent that all SNMP request for application_pending_request will be fired for it and it will return the queue depth. Why do I need to have an actual MIB when all I need to poll my application queue in order to get result?