Hi Ranjith,
they have exact same screens because it is all based upon the CRM Web UI so even you use charm u will be navigated to same screen of crm web ui.
Technically there are many things which will work in the same way but it doesn't mean it is design for that purpose..kindly check below link for documentation
Creating Damaged Test Case Support Messages - SAP Solution Manager - SAP Library
Technically you can create any custom transaction type and define that as per your criteria of status or action profile .....this is in the heart of CRM and you can do this from years ...but point is to understand why SAP has delivered different transaction type.....it is much like different document types like in ERP having different process designs.
Further, in terms of non technical difference it is from the Process or functional perspective
In simple words, SAP customers will know have the flexibility to have a different process for defect management and incident management.
hope the will help to understand better 
Regards
Prakhar