So that there is more dwell time for the bacteria, spores, etc to be interfacing with the UV rays that will kill them. More time exposed to the light = higher % dead.
I think its recommended to have a dedicated pump for these. Or you could plumb in a T with a valve and figure out the flow rate and get it down to 250 or so then through your sterilizer and then back to rejoin the rest of the output in the tank....