It seems counterintuitive for a carnivore to graze on the lawn, but eating grass is a common behavior observed in wild and domestic felids alike. It serves a digestive purpose.
Natural Laxative
Grass adds fiber to the diet, acting as a laxative that helps pass hairballs through the digestive tract. Alternatively, it can induce vomiting, which clears the stomach of indigestible matter like fur, feathers, or bones.
Safe Alternatives
Outdoor grass often contains pesticides. It is safer to grow “cat grass” (usually wheatgrass, oat, or barley) indoors. It is an excellent source of micronutrients like Folic Acid and gives them a safe outlet for this instinct.