Cats scratch to keep their claws in good shape, to stretch, and to leave scent and visual marks. The American Association of Feline Practitioners (AAFP) describes it as normal behaviour that owners should redirect rather than try to stop, and it opposes declawing as a routine solution.
Most cats scratch furniture because it's the best option available: tall, stable, in a prominent place and with a satisfying texture. Make the post better than the sofa.
