Surprisingly, cats eating grass is a fairly common behavior and probably not a cause for concern unless they're munching on treated grass.
If a lawn is treated with chemicals to kill weeds or insects, you'll want to keep your cat inside and off the grass. Eating treated grass and then licking and grooming their paws can make your cat very sick and may even be fatal.
When it comes to eating grass, experts have yet to link this behavior to any health issue. Regardless of whether you have an indoor or an outdoor cat, you should make sure that all your household plants are of the non-toxic variety.
Grass aids digestion
Cats regurgitate when they eat grass because they lack the necessary enzymes to break down vegetable matter. In small amounts, grass can act as a natural laxative, aid digestion, and prevent constipation. This is important because cats eat their prey as is, including both the edible and inedible parts which include fur, bones and feathers.
Grass contains folic acid
The juices in grass contain folic acid. This is an essential vitamin for a cat's bodily functions and assists in the production of hemoglobin, the protein that moves oxygen in the blood.
Grass is nature's laxative
Another theory is that grass acts as a natural laxative, counteracting any cases of indigestion. Cats are known to regularly throw up fur balls around the house. But when the fur moves deep into the digestive tract, kitty needs a little help to break it down and pass it out the other end. Call it kitty’s sixth sense, but your cat knows that a little bit of grass may just go a long way in cleaning out its system.
SOURCE
NATIONAL LIBRARY OF MEDICINE: “Characteristics of Plant Eating in Domestic Cats.”