Matching the flow rate to the sterilizer's "zap rate" or "kill rate" is how you get the most efficient use from one. Using more flow that it's rated for won't give enough radiation to actually kill the organisms that pass through it, and too small of a flow, is just inefficient. It won't HURT the sterilizer, but is kind of like using a shotgun to kill a mosquito...it gets the job done, but not very efficiently.
