Police are a deterrent. Their presence prevents crime. They also go after criminals, even if it's just after the fact. Take away the fear of getting caught and there's no reason for criminals to not commit crimes.

Of course that only works if police have the resources to actually investigate. Which, in liberal-run areas, they more and more don't. Thus leading to the situation the twat above complains about. Hint: the solution isn't even less enforcement.