Like people, cats need a little roughage every now and then to help keep their digestive tracts healthy. And cats actually enjoy eating grass and greens every once in a while for this reason. Some cats will seek out greens to eat and this may be why you keep finding your pet eating your plants or parts of your lawn. Please note that this can be dangerous or even poisonous to your cat depending on the plant he/she chooses to eat and if you use pesticides on your lawn.
You can purchase a cat grass growing kit at your local pet store. Some kits will have grass that is already sprouted that you simply maintain. Other kits contain the seeds and dirt and let you start growing from scratch. Whichever your choice your cat will probably enjoy a little patch of grass to munch on every now and then.