@Nandhini.Mani  it is not mandatory but highly recommended.  Starting with Pega Platform version 8.1, Job Scheduler and Queue Processor rules replace Agents and improve background processing.

More to the point, Job Scheduler rules replace Advanced Agents for recurring or scheduled tasks, and Queue Processor rules replace Standard Agents for queue management and asynchronous processing.

Please see the below available documentation on recommendations:

Background processing best practices.

Job scheduler FAQ

In Pega Platform version 8.1, job scheduler replaced advanced agents for recurring and scheduled tasks. 

Q: Will the agents in earlier versions of Pega Platform still work after 8.x?

A: Yes they will, but eventually you may need to convert agents to job schedulers.

Queue Processor rules

A Queue Processor rule is an internal background process on the server that you can use for queue management and asynchronous processing. You can use standard or dedicated Queue Processor rules.

Queue Processor rules replace standard agents. You can use standard queue processors for simple queue management, or dedicated queue processors for customized or delayed processing of messages. Queue Processor rules support higher scaling throughput than agents. Each queue processor can process messages across 20 partitions, which means that Queue Processor rules can support up to 20 separate processing threads simultaneously with no conflict

Replacing an agent with a Queue Processor rule for real-time messages

Agent to Queue Processor

Queue processor FAQ

Converting Existing Standard Agent to Queue Processor

Edit Queue processors and Agents to set up a system