why cats always do the scrathes in every furniture?  

And here's the long version:

"Cats are the only animals who walk directly on their claws, not on their paws. It would be like humans walking on the tips of their fingers. Getting around on tiptoe, called digitigrade, is a particularly useful feature when it comes to moving at high speeds. Being the great hunter that they are, they need to keep this skill very sharp. Contrary to popular belief, manicuring their nails on your furniture is not out of malice or sharpening the claws. Cats are tearing off the ragged edges of the sheaths of their talons when scratching. All year long, they shed their claws to expose new sharp ones beneath. Claws aren't just for protection. They are responsible for the cat's exquisite balance and amazing feats such as climbing, stretching, running, and grooming. Declawing not only physically afflicts a cat, but he is also psychologically anguished, deprived of his only defense and one of his most versatile tools for survival."