Just wanted to chime in to say that in SF if you have a parking permit you can park in some spaces for up to a week until street cleaning. When I had a car I would have gladly paid someone $15 to get a spot on my street on a Thursday knowing I could leave it there until Wednesday. Spots open up all the time but are filled quickly. I'm not defending the app, but there's definitely a (small?) market for something like that here.