Two reasons of belief here from talking to my vet. The first one is when dogs seem to eat grass really fast without chewing they either have something stuck in their throat or an upset stomach. When they eat grass that quickly it allows them to almost instantly vomit which settles their stomach or removes the dislodged item in their throat.
Second reason she said was that some dogs just enjoy it. If they nibble on grass and actually chew it and only eat a small amount more than likely they won't get sick they just enjoy the taste of it.