Cats are predators. And they don’t lose this instinct even when they live their whole life indoors. As being predators cats have a huge urge for motion, such as running, jumping and climbing. Every cat owner surely can tell a thing or two about it.
Also as being predators cats need to sharpen their “weapons”, meaning claws, from time to time. Cat trees meet both of these needs in the most perfect way – and it doesn’t matter if your cat is an indoor cat or has the opportunity to go outdoors.