Yes spiders do do all the things you mentioned, far as I know.
I think the case in point here though is esthetics. They're not talking about a few neat, fuzzy cobwebs in a corner or two. In fact, they're not even necessarily all spider webbings; some are just spore growths from things that float in the air. In the country especially the cobwebs will often cover the entire north wall of a building, especially if there is vegetation along it any higher than grass. Then it starts collecting dust, insects, dead and/or eaten, pollen, spores, anything light enough to float in the air. Especially after a humid summer as we've had here lately, first the side of the building begins to look a little dingy, then gray, and then more black than anything else by the end of the summer. My sister next door has one of the most text-book examples of it I've ever seen this year. She tried sweeping and that got rid of the fuzzies but not the black film. She tried washing but that was a huge task, too huge for her. We've found the pressure washer does the best job of cleaning and also has enough force to remove the black from the vinyl without damaging it since no soaps have been needed. Yet, anyway. I normally wash down my siding a couple of times a summer and it seems to suffice. No chemicals, soap or anything else necessary.
HTH, PopS