Cats' nails grow continuously. Therefore, the old nail sheaths need to be removed and discarded. Cats accomplish this by scratching.
Scratching keeps their nails in healthy, tip-top shape.
Most cats will gladly use scratching posts designed for that purpose. Several posts are usually needed, distributed around the house. Unfortunately, some surfaces are just too good to go untested. Most cats like rough textures. It's wise to provide cats with scratching furniture that gives them a choice of surfaces and heights.