Think about how much more aware of our surroundings that we are when we're on a bike as opposed to in a car. While cycling, I am very aware of my surroundings. I believe that it is the same way for LEOs. It is also a good visible presence.
My wife and I were in Chattanooga for a few nights several years back. While out on the patio of a downtown restaurant, I counted something like 5 cop cars with their lights going for no reason. I asked our waitress "Why the light show?" She told us that groups of "hooligans" would come into downtown and cause problems, so that was the cities response to the problem.... Squad cars with lights running.