Professional players need to keep a consistent weight to their blade. Perspiration can cause a handle to become heavier by 5-10 grams within a year. This changes balance and feeling of the blade. Sealing your blade, especially the handle, helps keep the weight of your blade intact.
Besides, it also helps prevent delicate blade surfaces such as hinoki surfaces, from splintering when removing your rubber sheets.
Apply 1-2 thin layers of lacquer evenly to the blade surface with the included reusable/washable sponge.