there are various reasons why dogs eat grass. it is a normal part of their diet because wild relatives of dogs (wolves foxes etc) eat rabbits and such who eat grass, and therefor when theyre eating the rabbits squirrels etc, theyre in turn eating the grasses in theri intestinal tract (gross i know:P) Another reason is if they have an upset tummy theyll eat grass to induce vomiting it acts as an irritant and causes the dog to throw up--it also binds with whatever matter the dog is trying to puke.
Some dogs just like it.. others will run the blades through their mouth to gather info via smell/taste.