Its the best thing I ever added to my chickens run - apart from the chickens that is!
After the ate all the grass the area was just dirt. When it rained it would get muddy and the chickens would also get very dirty, then go in to roost and make everything inside muddy too.
At first I added some sand, but it was too fine, and it trapped the moisture and make the run smell - something that had never happened before.
So I took it out and added a course sand. That is perfect. It drains away quickly, there is no smell, the chickens love scratching about in it and eating some natural grit.
Also I have noticed the chickens feet are very soft now and the claws are kept shorter as they scratch about in it.
I do not it in the coop, only in the run.