Why do cats scratch?
by KittyKondo.com

Its very natural for cats to scratch and they have a very innate behavioral instinct to do so. Scratching serves some important functions:
   First, it helps to remove the outer surface layer of the claw so that it becomes sharp and new. This behavior is vital and plays out the cats natural predatory instincts.
   Scratching also spreads secretions from glands between the footpads that signals other cats that the scratched area has been marked. Indeed, if a cat scratches in front of another cat it is being assertive and marking out its territory.