I'm not too sure about UHF "in game" since I just started playing this sim, but as far as real world...
UHF is used by military aircraft to communicate with ATC because there isn't all the airline chatter on UHF like VHF. Of course, sometimes it's hard to get the ATC controllers to hear you because they monitor both VHF and UHF.
Also, most comms to other aircraft and ground bases is on UHF.