Each Application Pool runs with a single worker process (w3wp.exe).
From IIS 6, we can assign multiple worker process on a single application pool. This method is called Web Garden.
Many worker process can provide better performance and response time to your application.
There are some restriction to use web garden for your application. You must use "out proc" for Session Mode and "Session State Server" or "Sql-server session state".
If you use "in proc" for Session Mode your application not work correctly.

To create a web garden, right click on the Application Pool -> Advance Setting and set the value of Maximum Worker Process under the Process Model section.