As a mobile detailer, I can tell you they do a lot more than just block some of the sun. Perception is sometimes a great asset when dealing with clients. If the canopies didn't block a lick of sun, it still looks more professional than just detailing a car out in the open. What sets you apart visually from the guy next door detailing his car while you're detailing a clients car. Sure, he could have a canopy himself, chances are he doesn't. So while the canopy may not block out the sun that day, I'd still set it up just to look more professional. Sometimes image is everything, isn't that one of the reasons your clients want to look good in their cars.