The general line of thinking is that if you make it easy and free to park everywhere throughout the city then people will have no reason to consider using their cars less and consider other methods of getting around (I'll leave others to debate whether those other methods are viable or not). There's also an argument that large parking facilities are a waste of space and resources that drive up construction costs, chew up land area and produce little to no economic benefit in and of themselves. There's also a particular school of urbanist thought that takes issue with so much of the design of our physical environment being dictated by the maneuvering and storage needs of private motor vehicles ("forget the damned motor car and build the cities for lovers and friends"). These are all value judgments of course and can be debated in many different ways but those are the major points, I think.