I agree, it will not help much if your tank is already infested with ich.. It will only reduce the amount of ich that are free swimming in your tank...
If your tank is matured and yet the fishes you added still get ich, It is advisable to let the UV run for 1 month before adding new fishes to let the ich finish its cycle and hopefully, during the free swimming stage, some will get killed by the UV when they pass thru the UV...