at night it is for DUI paranoid drunk people watch for cops in the mirror, you drive out of yer mirror you swerve ( thats why you turn them up and focus)
...like fuzz said it is to make you give them a reason to pull you over. Trust me no one wants to be behind YOU that long for no reason... the smell is horrid
, During the day it usually is to wait until you are in a safe area to pull you over.