The "Requested Role" is important when the project came from a request. When completing a project request, the requestor has the opportunity to request functional roles instead of individual resources. When the reviewer is converting the request to a project, these requested roles will be staffed with actual resources. The requested role column shows what the original role request was from the request phase.
If the project did not come from a request, the requested role will always be the same as the primary functional role of the resource being scheduled.
Unfortunately it is not possible to schedule resources on a project by anything other than their primary functional role.
I hope this helps!