I would hypothesize that a cover would restrict air flow around your sweat glands which would reduce your scent on the air around where you're sleeping. It woul

I would hypothesize that a cover would restrict air flow around your sweat glands which would reduce your scent on the air around where you're sleeping. It would also mask the outline of your body. For insects adapted to bite humans, hiding these signals and providing a physical barrier would possibly reduce bite incidents. Reducing bites from potentially pathogen-spreading insects would have an evolutionary advantage.