Very simply put - there would be a lot of "mael" on that knob from combining of dead skin and skin oils. Remove and wash with some saddle soap (proper mochi aka cobblers have it - not rackety shack cobbler at the end of your galli). It can really clean skin oil etc.
Now your options are to either buff it clean and use as is or stich a leather cover for it - stiching the cover is quite an art and also requires the use of steam to help stretch the leather onto the knob. When done correctly with double needle stich - it looks and feels really good.
