Why do police officers leave their cars idling?

I just asked some and they said they have equipment in their cars that needs to be on at all times. Is this true? What equipment? Why? Wouldn’t it be cheaper to just get a battery? Gas isn’t free.

I told them it was bad for the environment. They looked stunned and told me smoking is bad for the environment too. Then I looked confused and told them I don’t think anyone should smoke, either. It was not a very productive exchange.