I think it's to keep people like the ones on my street from doing what they do. There's a car from NY, one from VA, and one from Utah. I see each of these cars all the time, and they rotate them in and out of the parking spots in the driveways so that the cars don't get ticketed.

Interestingly enough, the one from VA says TIE FTR. That's about the only reason I haven't called 311 on it.