Usual argument is that it prevents vehicles (sometimes with less disciplined RTF usage) blocking aircraft transmissions. Depends a bit on how busy the airport is and the frequency configuration but in my view ideally there should be complete cross-coupling on GMC for vehicles on the manoeuvring area and transfer to AIR frequency for vehicles crossing or operating on runways. When I last had any involvement with this the CAA expected the airport operator/ATS to define what they wanted and to argue why this was most appropriate so each airport comes up with a slightly different arrangement.