With wear, your waterproof garments will become less effective. That’s because they get dirty. Even if it’s not visible, the layer of contaminants on the garment’s surface causes the DWR treatment to stop working as well, so that the outer surface of the garment will get saturated in the rain and water won’t bead off.
Dirt, oils and sweat from the rider will coat the inside of the garment too, partially blocking some of the pores in the membrane, meaning that there’s less effective removal of water vapour from inside the garment. The result: you’ll keep less dry as you ride.