Why does your cat eat grass? This behavior probably dates back to ancestry. Cats in the wild would consume prey consisting of mostly herbivores, such as mice. That meant they also ingested whatever their prey ate – primarily grass and plants. Grass eating in today's world might somehow satisfy an appetite for them.
While in most cases grass eating isn't a problem, you should make sure that your cat is not consuming chemically treated grass or toxic plants. Cats are notorious for chewing on leaves, such as lilies, and this can cause life-threatening illness.
If you catch your cat chewing on a suspicious plant, contact your veterinarian as soon as possible, as early treatment is crucial.