Cats: Scratching Furniture: A Common Behavior of Even Declawed Cats by Holly R. Frisby, DVM Veterinary Services Department Drs. Foster & Smith, Inc.
Cats "scratch" on items for two reasons. The reason we think of most often is to sharpen their claws. But this scratchng also serves another purpose. When scratching on items such as furniture or scratching posts, cats are also leaving scent from the sweat glands on the pads of their paws and scent glands between their pads. The scent left behind says, "I was here, and this is my territory." Once their scent is on an object, they often return to the object and apply their scent again.
We can use this information to our advantage when trying to get a cat to use a scratching post. If we can get a cat's scent on the post (put a little catnip on the post to get the cat to scratch), the cat will likely come back to the post to replenish his scent. Similarly, it is helpful to remove the cat's scent from objects the cat has been scratching on but we wish he wasn't.