Since dog’s do not talk we can’t really know for sure. I know that’s not the answer you wanted so all we are left with a couple theories.

1. They eat grass to throw up! Yes it’s disgusting and they do it but why? It could be left over from being in the wild. If they were to eat something that were to make them sick they instinctively induce vomiting to rid themselves of what they shouldn’t have eaten.

2. Another theory says they eat it because they like the taste and the fiber aids in the digestion process. Some dogs do actually eat a little bit of grass and do not throw up. If your dog eats grass and does not throw up there’s probably no reason to stop them from doing so.