MARS & CAP officially refer to Military Auxiliary Radio System (MARS) and Civil Air Patrol (CAP). Both use frequencies outside the ham bands in either the VHF or UHF band.
The modification allows you to expand the transmit frequency range of your radio.
*Not all radios are capable of the modification*
Interoperability.
I worked on a joint exercise with county Fire/EMS, CERT, and ARES. To fully communicate with each other, we utilized iterant frequencies (with permission) to coordinate search teams and county government officials. Without having the MARS modification, I would not have been able to program those frequencies into my radio.
During our exercise, a real medical emergency occurred, and I had to relay radio traffic for the Fire Battalion Chief to medical personnel. His county radio was in a dead zone, but I could hear him from my position and become a relay for him.
I'm not a lawyer; this is not legal advice.
You do not need the MARS/CAP mod as a new ham. As you progress in your amateur radio journey, joining an EMCOMM or AUXCOMM organization, you may want to look into this modification for one or more of your radios. Your local EMCOMM or AUXCOMM group can best advise you on the best course of action.
Getting this mod to transmit on GMRS, FRS, or MURS frequencies is not a valid reason in my opinion.