Have you ever stopped to think about the rain gear that you bring on backpacking trips and its true purpose?
For example, there is a common misconception among many hikers that the purpose of rain gear is to keep you dry. Nothing could be farther from the truth. Instead rain gear is meant to keep you warm when it's raining by creating a vapor barrier around your legs and torso that keeps your body heat trapped close to your body. In doing so, rain gear will often make you as wet from condensation as if you were naked in the rain. Except without it, you'd quickly get chilled and possibly become hypothermic.
The same goes for pack covers. More often than not, they do little to protect your pack from rain and instead cause so much condensation to occur that your pack's exterior gets wet anyway. In reality, it's your backpack liner that keeps your gear dry. A pack cover's real utility is to protect your pack from damage from stray branches or thorns, particularly if you have an ultralight pack that has a lot of external netting. I keep my pack cover on most of the time these days when I'm hiking rough trail in Vermont or New York State to keep it from getting torn, and it's a must have when bushwhacking.