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.
#2 The roughage theory: Because dogs need some cellulose in small quantities to aid in bacterial digestion (sort of like a prebiotic), they sometimes consume the green stuff by way of reestablishing bacterial homeostasis.
#3 The vomiting theory: To raise a little bile when something didn’t go down just right, the nausea-inducing properties of grass are well-documented. Just eat a handful yourself if you don’t buy this logic.
#4 The angry tummy theory: If you had some excess gastric acid, maybe a little esophageal reflux and no prehensile thumbs with which to grab a bottle of Tums or a little Pepcid AC, you might just eat grass, too.
#5 The hungry dog theory: Some dogs just like it. Period. We all know a few who doubtless do.
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.