It re-transmits on UHF so they don't have to keep switching radios to talk to disimilarly equipped aircraft.
IE: Here in Charleston, VHF tower is 126.000, UHF is 239.000. It can also work on VHF to VHF as 118.0, 127.325 and 381.6 are the clearance delivery frequencies, and the tower re-transmits all three of these on ground- 121.9, which can also be a clearance del. frequency.
edit: One funny note about the "Galaxy" callsign, (btw I work at KCHS/CAFB). I watched a C-5 land one morning, roll out and the TX between the tower and the A/C went like this:
Tower: Galaxy xxx, make right turn on alpha, taxi to the mil side
Cutest little squeaky girlie voice you've ever heard: Right on alpha, taxi to the ramp, Galaxy xxx.