Outdoor pens for cats?

I don't really find that I need this since my cats are able to run around the backyard, up trees and other things. They also don't wander far from the main house so it never occured to me to build them a play pen outdoors. I find that if you let them run around when they are still kittens they tend to explore nearby areas but only gradually and later on they stick to staying in the immediate area of the house. I suppose this building a pen is great if you live in an apartment compound.