Really, the main reason for cats to eat grass if to help them in regurgitation of food (or hairballs).
Feral cats will eat grass in the wild between live animal feedings (to help regurgitate the indigestible parts).
Indoor cats would use it to help with hairballs or other issues with digestion. They do not really break down the grass- it is more like fiber and aids in the regurgitation process.
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