As if You were adding a New Hire. Say there are 6 or 7 different types of 'roles' in your company.
If you chose the dropdown 'CSR', the person making the new hire request wouldn't have to choose anything else besides a name and start date.
If it were a New Hire in Marketing, the Marketing Dropdown would bring more fields into the equation, such as: Memberships in AD, or if they need a laptop/cellphone, etc.
In essence, the dropdown would auto populate the incident/request.