Technical waterproof fabrics like those used in dry cags and trousers only do their job effectively (keeping you warm and dry) when water doesn’t soak into them. Through wear and tear this ability to shed water begins to fail leaving you with kit that soaks up water. This often means that your kit takes on extra weight, takes longer to dry and is not as breathable as it may have once been.
Dry tops and dry suits as well as other “dry” kit are treated with a DWR finish. DWR stands for Durable Water Repellency. This is the treatment that makes water form as droplets or beads on your kit. When this starts to wear off (which it will) your kit will start to soak up the water. This may not make it through the laminated layers of the fabric but it will comprimise the breathability and effectiveness of your kit. This is known as “wetting out”.