I don't know the reasoning of DMV. But I think that the ability to effectively parallel park is a clear demonstration that the driver knows where the wheels and the four corners of the car are located, and how the vehicle actually turns. If you can parallel park, you should certainly be able to park any other way. And if a car parks very close to you in a parking lot, you have the skills to be able to get out of your spot without hitting the adjacent car. An understanding of how the wheels work in parallel parking can be applied to other close maneuvering: one should be less likely to cut a corner too close and hit the curb, for example. Parallel parking requires a good understanding of close maneuvering which can readily be ported to many day-to-day situations.