Cats seem to enjoy eating grass, and there's no absolute answer to why. Your kitty is an obligate carnivore—that means she requires meat to survive and does not rely on vegetables or fruit in the diet. But some kitties never read the definition, and they occasionally eat grass, flowers or other surprising things.
Why Do Cats Eat Grass?
Wild cats eat more than just the flesh of critters. There's not much on a mousy drumstick, and the partially digested plant or grain material in a victim's tummy offers nutrients that cats readily use.
Other Possible Reasons
There's some speculation that eating grass provides trace elements of vitamins that cats instinctively seek. It may also help provide fiber to move out hairballs or act as an emetic to clean kitty out from the other direction.