The first time you got caught in a heavy downpour and had to ride many miles through it, you probably learned this tip the hard way. The stuff in your backrest bag undoubtedly got soaked. Even if the bag is waterproof (which few are), the water has a way of seeping in through the seams and zippers and trickling down to your clothing. Many backrest bags come with a waterproof rain cover, but by the time you pull over to put that on, the water may have already seeped in leaving everything inside wet.
I never leave on a trip without first lining my bags with a plastic garbage bag. It takes up no space and ensures my clothing stays dry should any water seep into the bag. I even do it with the items I stuff into my saddlebags. I put everything in those plastic grocery bags should any water seep into the saddlebags.
Colorado Springs, Colorado