I guarantee that most of those tarps are covering up leaky roofs, but a few are certainly for winter storage.
Fact or fiction, some claim that the tarp traps moisture and causes mold growth.
I've read where people have used tarps and not had any mold problems, but I have never heard someone say they tarped their RV and had mold. Maybe they are just too embarrassed to come back for the "I told you so" or maybe the mold thing is just a myth? I don't know.
Tarps are definitely rough on paint. I tarped a load in my pickup truck for a quick slow speed trip across town and it scratched the heck out of my paint.
Tarps are tough to get (and keep) tight. Loose and flapping will wear a hole through on anything resembling a sharp corner in short order.