I've heard that cities also charge for parking because of an interesting social factor: many people will drive in circles for half an hour looking for a free spot rather than pay for parking in a private lot. Everyone driving around in circles creates more traffic. So the city charges for curbside parking, in order to make the cost more comparable to parking in a private lot, with the hope that it will lower the psychological switching cost from "finding a curbside spot" to "giving up and paying for a garage spot".