no idea!!! I guess this is one of the biggest dog behaviour mysteries ever. All the dogs I ever had ate grass. One of my current mutts loves to tuck in after we come back from a walk. But I've noticed that everytime any of my dogs had an upset tummy they always headed for the grass. It has been suggested that grass is a natural emetic (induces vomiting) so maybe it makes their stomachs feel better. But my daft dogs eat it when they're well too...maybe it gets them high
We even had a cat who enjoyed a good chow on grass, although I think she just copied the dogs as she always followed them around and joined in with any mischief they were up to.
I've never seen any harm in it and allow them to do it, but only in my garden, never in a public place in case they pick up parasites from other dogs poop that might be lying about!