Use case 1: We want to leverage the Business Case and Portfolio Optimizer on requests as well as projects to determine whether requests should be approved.  
Use case 2: We want to use custom fields AND the business case.  We use a request to solicit required custom fields.  This request prevents us from being able to use the business case and portfolio optimizer.  Alternatively, if we go directly to a project and leverage the business case, and we lose our required fields.
Use case 3: Requesters submitting the request need to provide the business case details and may not have a WF license.