Sometimes it's nice to have a portable nesting box. If you free range, you can put it out in the yard where your flock has decided they are going to lay their eggs to try to encourage them to lay in the box instead of under a bush or on the ground.
In the summer our coop is sometimes just too hot inside for our hens to want to lay their eggs, so I have some baskets and wooden boxes out in our run where its cooler. Maybe you have an injured hen or one who needs to be separated for some reason, so she doesn't have access to the coop nesting boxes. A broody hen needs a safe nest away from the general population where she can hatch her chicks and raise them.
In all of these cases, it's nice to have a nesting box that you can move to where you need it. You can make this cute 'box' in about ten minutes out of a wooden half wine barrel.