Probably in situations where the dude is just driving around with a gun and shooting people (as rare as that is). Probably want to stop the guy instead of giving him a ticket later after hes done killing lol.
Honestly, I understand why he shot. The cop came in a bit hot at first but he didn't immediately use a gun. It only came to that after Lenny started winning the brawl and after attempting to use a Taser instead.
He even started to apply first aid afterwards. So I think the cop did okay here.