One of the 5 P's in a marketing strategy, "price" can be viewed as a tool that affects how prospective consumers perceive the value of an offering.
For instance, there is no direct cost to using a public park but there are costs like maybe having to manage less privacy or congestion or cleaning-up after yourself. As a result, public parks may not be perceived such a good deal even though "free".
Nonprofit health centers offer a sliding fee scale to accomodate varrying income levels and needs. The down-side is that a wider number of consumers may be missing the boat in what is generally high-quality care because they perceive that providers offering a sliding fee scale must be providing a lesser quality of care.
That said there is a good article in Governing Magazine on the role pricing plays in the world of parking, particularly street parking, and an experiment in variable pricing which San Francisco is going to test. The basic premise: price higher where there is bigger demand. Price lower where there is less demand.