Why do domestic cats eat grass?

From When My Cats Are Eating The Grass.

Keep off the grass!

Outdoor cats eat rodents & birds. Therefore, outdoor cats eat processed grains, seeds and nuts. Wait, WHAT?!?! Rodents & birds are grain, seed and nut-eaters, though some birds eat worms. Therefore, domestic cats are eating grains, seeds and nuts that have been chewed/pecked and swallowed i.e. processed grains, seeds and nuts.

This may be one reason why domestic cats chew grass - to get chlorophyll to "neutralise" the iron in the meat. Another reason is to vomit up the indigestible parts of rodents & birds that the cat ate, including fur-balls. There's also the Folic acid angle.