Cats often love a bit of grass to chew too. It can help with their digestion (or as is so often the case help them throw up) Our cats go wild whenever we come home with a pot of cat grass, and the all take turns having a bit, or in the case of some our nuttier ones, grooming it.
in the wild a cat would get 3-5% greens mostly from prey stomachs ... so it is needed in a small amount.... FYI in most formulas greens being 8-12th on ingrediant list would put them in that ball park