Scratching is an innate feline behavior, and it is something that all cat owners must learn to deal with. Cats scratch for several reasons. First, scratching keeps their claws in shape. You schedule a weekly manicure, your cat schedules a scratching session with the sofa. Not only does scratching sharpen the claws and remove the old outer husk of the nail, it also feels good.
Scratching also serves as a territorial marker.
Cats may scratch in preferred sleeping spots or any other place where they spend a lot of time. Doorways and window sills often get scratched, especially when an indoor cat spots an intruder outside. He scratches in a vain attempt to let the other cat know that this is his territory. Scent is another aspect of territorial scratching. The sebaceous glands in a cat's paws leave an odor at the scratched area - another way the cat can stake his claim.