Imagine two apartment buildings that are identical in every way except one. Building A has its parking in a garage that sits within the building's footprint and Building B has its parking in a surface lot. Which building has more residential density (dwelling units per hectare)? Clearly Building A. Density is important for supporting convenient transit and habitat conservation.
Usually the decision about whether to build off-street parking is a foregone conclusion, since most zoning codes require it (sadly). The way in which it's built is usually based more on the cost-benefit calculations of developers. Where land is cheap (say in the 'burbs), it's usually more advantageous for a developer to build a surface parking lot than a parking garage. Where land is expensive (say within an urban center) the developer usually comes out ahead building a garage and economizing on land.
This is an important point about suburbia, because suburbia often does have apartments in it, yet their parking areas are more likely to take up land in addition to the land the building sits on. This means you get less "bang for the unit" in terms of density.
The moral of the story is that expensive land is a friend to urban life. Environmentalists are always pushing for putting a price on carbon to get people to reduce the use of fossil fuel energy and promote sustainable alternatives. They should also think about the positive effects that expensive land can have on urban design. When land is expensive developers conserve it in their quest to maximize profits. They build small-lot houses, rowhouses, apartment buildings and parking garages (when allowed to) instead of large-lot ranch house subdivisions and oceans of surface parking. The key is to allow for some density and relax the parking requirements where land is expensive so that we can have the efficiency advantages of expensive land without killing off the prospects for affordable housing and affordable tenant spaces for small businesses.