A waste of a perfectly cromulent parking space

So, I was walking home a couple of nights ago, and noticed that a few parking spaces down from my front door was a "booted" vehicle. [I must admit, in spite of the outrage, that it was interesting to see what the boot looks like--and how the depiction of it on The Simpsons all those years ago was so interesting.]

But in any case, the annoying part of me was thinking, "Why would you waste a perfectly valid parking space by booting a vehicle?" Parking in Cambridge (and the entire greater Boston area) is already difficult enough. Why make it any more difficult by booting vehicles in place? Why not just impound them?