Keep your tubes in the best shape possible and avoid one more potential puncture issue.
 
Stop the dirt and water from entering the rim through the valve stem hole. Over time, the grime getting in forms a paste that will work like sandpaper between the rubber and tire. Eventually, it will damage your tube, and the water will rust up your spoke nuts from the inside.
 
When mounting a new tire or tube, it's always nice to pamper your tube with talcum powder to keep it smooth and silky. It will age better that way, and the valve seal is a simple low-coast piece to keep everything clean inside your wheels.