Yeah, it's not eating vegetation but digesting it.
It's why carnvores always eat the stomach of herbivores first.
All the minerals and vitamins in that pre-digested plant matter.
Dogs mainly seem to eat grass for medicinal reasons.
Which seems odd to us.
But then americans eat puke flavoured chocolate, which seems weird to the rest of the world
But maybe our ancestors also ate the paunch of herbivores and thats what americans see in butyric acid flavoured chocolate.
There was a woman who planted a field full of medicinal plants, and her horses would go into it and self medicate.
They instinctively knew which plants to eat.
Cut right down on her vet bills
No doubt dogs have similiar instincts.
Ours seem mainly to eat grass to make them throw up.
Except our old spinone ruffy, who could selectively throw up anything in his stomach.
He'd swallow stones as an easy way to carry them and then just throw them up - and nothing else - when he got home.
Our drive is still lined with stones (small rocks really) that ruffy carried back from walks - either in his stomach or mouth.
He could also carry 4 tennis balls in his mouth