When you keep more than just a few chickens having some extra pens is essential.
They have lots of uses but having somewhere to keep a poorly chicken away from the others is essential and a pen around this size also makes for a great broody coop for a hen to sit on some eggs and have the chicks have safe access to some grass outside.
This chicken tractor/ark is one I built in 2008. The picture above is how it looked when I first built it, complete with cedar cladding in our very tidy garden at our first house!
The picture below is me putting our first ex-battery hens in the coop and my first chickens since leaving home.