The Need to Scratch
Cats scratch for a multitude of reasons. To mark their territory, to shed/file their claws, to relive stress, and to stretch their bodies.
When a cat scratches an object they release a smell that is their own onto the object stating that "THIS IS MINE, NONE OF THE OTHERS CAN COME HERE!" Its a smell only noticed by other cats.
Cats nails/claws grow in layers and eventually they need to shed the outer layer called a "sheath". Have a good scratcher promotes healthy nail growth, and makes it easier for the cat to do its thing.
Cats will also scratch to relieve stress. Its a soothing action that releases a smell that is them and its the equivalency of purring. Ever notice if you get home late and the cat goes right to the couch and has a go at it?
Cats also need a place to stretch. Having a post allows them to dig their nails in deep and stretch all those muscles in their bodies. Cats have 230 muscles in their body, compared to the 206 in humans. They have exactly 32 different muscles in EACH ear, allowing their helicopter like mobility. With that many muscles, wouldn't you want to stretch too?
What Kind of Cat do you have?
The Vertical Scratcher
These are the cats that like to stretch out and claw up the furniture. They like to destroy the drapes, the walls, etc. (thus providing them with a healthy alternitive)
The Horizontal Scratcher
This cat will ruin your carpet from one end to the other. They paw at the ground obsessivly.
*Most cats are a combination of both. Get a variety to see what your cat would use and continue to use*