Dogs do it, that's all. Their dams ate their faeces when they were pups[ it's conceivable that it is 'puppy behaviour', imitating that. Some dogs acquire the habit when they are grown. Some of mine , but not all, do it, which suggests that it is not compensating for some dietary deficiency since they have the same diet and all are healthy. I think they just like it; mine are quite discerning, in that each has its preferred material.