They allow your cat to go down from the cat flap in the window to ground level or straight up to under your house eaves, cats really love to be up as high as they can. The runners are also used to link the Cat Enclosure to your house and linking 2 or more Enclosures together will give your cat room to play around in a safe environment. The Tunnels join together to make any length that you require. A great combination is to have 1 Enclosure near the house and have some runner to an enclosure that is down through your garden, this way you don't need a large Cat Enclosure. You can have a couple of small sized enclosures using the Vertical Runner to link them to make a big cat run.