Grass does not provide a lot of nourishment for cats, but grazing is a necessary part of cat's life to keep them healthy and happy.
Grass contains mainly fiber which is indigestable to cats since they do not have the enzyme needed to break down the fiber. Cats consume grass for mainly several reasons:
Induce Regurgitation
As disgusting as it may be to deal with cat vomit, sometimes it is the best way for cats to expel indigestable substances from their digestive system. Outdoor cats may prey on rodents which contain bones and hair that cannot be digested by cats. By nibbling on the grass blades, cats can induce regurgitation and remove the indigestible material out of their body. It is perhaps the most desirable way for them to get rid of the inert substance instead of pushing it through the intestines, possibly causing abrasions. If food is not digestible and has bundled up in the intestines, it may require surgical procedure to remove the blockage. That's why grass consumption will be much better and a more comfortable alternative for cats.
Indoor cats will most likely stay away from rodents, but since grooming is part of cats' daily routine, they scoop up loose hair from their coats and it may accumulate into hairballs. Hairballs is a very common issue that cat owners face especially often during shedding seasons. Brushing your cats' coats can help reduce hair consumption in cats.
Nutrition
Though grass does not have a lot of nourishment for cats' daily needs, it does provide certain minerals, vitamin A & D, and chlorophyll which boosts up the immune system and helps prevent many infections and diseases. Finally, grass has a large supply of moisture which provides water for cats to quench their thirst.
Taste
Even though your cat is healthy and in no need of any fiber foods, your cat may still go for a few blades of grass because it stimulates their palate. Grass has the taste that cats enjoy very much.