imagine sweeping a floor
you do one sweep across, get most of the stuff... oh wait there's still a few pieces of lint.
do a second sweep ur good it's perfectly clean.
hell man, the only reason I do 2 wipes is because sometimes the ROM developers specifically say we should do at least 2 wipes. I figure they know better than me, and even if it's unnecessary, it's not like I'm going to regret spending the time to do the second wipe for the rest of my life or anything o.O