1. they enjoy it (kitsu almost looks zen munching on grass!)
2. they have to barf (expel excess mucus, bile, and other impurities)
I also have heard that there are other benefits as grasses act as an internal cleanser and a sort of herbal medicine.
Dr. Patty of Dolittler/Fully Vetted gave these reasons:
#1 The vitamin and mineral theory: As in, they’re not getting enough, therefore they consume grass.
either way I like them to have it if they need it and I planted some organic 'cat grass' for them this year because I can never guarantee that the grass in our yard is worm guts/bird poop/chemical free.