Is eating grass normal or abnormal for your pet ??

I thought that when an animal would eat grass, it meant that they were lacking something in their daily nutrition. But I didn't know that grass meant that they are having an upset stomach. That's news to me. I know that when my dog was alive, he would randomly eat grass. At that time, we would feed him something filling and filled with different nutrients such as wet dog food and he'd be fine afterwards. I wonder what makes animals go to grass in the first place? It seems so normal to be eating a plant, you know? It would make us much more aware if they were eating something a bit more unnatural to the eyes such as dirt.