most often it is to get rid of hairballs which can both sit uncomfortably in the stomach, and inhibit normal digestive processes. Often the reason they eat it last thing at night is because they have spent the day cleaning themselves and then when you let them out before bedtime they think oh that might be a good idea! unfortunately most dogs dont even wake u up 2 let them out they merely leave it in the middle of the floor so that you walk in it in your bare feet when late for work!