I guess you don't have tree sap, pollen, ashes from fire pits, etc. where your ride, camp, adventure out etc.
I like to protect my investment and find removing tree sap a real pain in the ass task. I rather it hit the cover rather than the bike, I do try to park away from trees when I can but living in New England you don't always have that pleasure. I summer up by a lake here in New Hampshire spend at least 4 days a week every week there and the only parking area is canopied by crappy pine trees. Hopefully this makes you understand the need for a bike cover.