All cats, big and small, eat a certain amount of grass to clear their gut of inedible material like hair and bones from eating and cleaning through vomiting. Cats don’t produce the right enzymes to fully digest large amounts vegetation. The juices of grass have folic acid and some vitamins that wild cats mainly obtain through eating grass. The fiber from grass also acts as a laxative, very good for passing old hair balls. If an indoor cat can’t get to grass it might turn to houseplants as an alternative, but many house plants are very toxic to humans and cats.