How to automatically populate custom field of Defect work item by Roles

Hi,

I am configuring defect work item. I want to create a drop down box which will automatically populate Role that I have created for Project area(e.g. Tester , developer) . I basically need to understand what is use of Roles that we have created  apart from just assigning Permissions specific to roles . If Defect is created by Tester then Defect Work Item should have a field which will automatically show that tester has created it.

Is it feasible to do so? Or do I have to manually put values of roles in drop down list.