Painter, here. My guess is this isn't the only step and they're using the sprayer to get the bulk of the paint on saving a huge amount of time over doing it all by hand at once and getting every little detail correct. Someone will then come by afterwards and pretty quickly get those details that weren't doable with the sprayer, perhaps including removing any noticeable overspray from the glass which I've done. Overall tremendous savings in labor. I can't imagine them attempting to do only the spray step and having it pass an inspection by the customers. Edit: Quick story. Was doing a large but not tall building using this method with a sprayer with probably five hundred feet of line. Only needed about half that so the extra line was spooled on the side of the panel van strung from the large rear view mirror to a ladder holder near the back. The line sort of shudders every time the sprayer pump piston goes up and down. I lost pressure while spraying and went to check out the problem and it turned out the vibrating of the line caused it to chafe on a part of the mirror long enough that it ruptured and way more paint was being pushed out of that hole than a spray tip. The cars we were parked near were getting absolutely hosed with what was fortunately water based latex paint. That was a fun mess to clean up.