Why do cats scratch?

Exercise
First of all, scratching is great exercise! Do you like to stretch when you wake up in the morning? Your cat certainly does! When cats scratch, they stretch out their back, legs and paws, and do a little pampering – scratching to file, sharpen and clean their claws.
Territory
Cats like to feel relaxed in their own space, and use a number of methods to communicate their territory to other cats. Scratching is one way for your cat to make their mark on their home, signal their presence to other animals, and feel happy knowing that their territory is safe.
Instinct
The natural urge to scratch is strong for cats; if a surface looks and feels good, they may find it hard to stay away! Because of this, it’s important to respect your cat’s instincts. Ensure their needs are met by providing alternative scratching options!

Your cat will naturally seek out a range of places to scratch – so you should never try to stop them scratching completely.

excerpt: feliway UK blog