Question on cat eating grass****

I have a question for all you knowledgeable cat people. My male Hobbs was taken in as a stray when he was about 6 months old. He is about 5 now, and generally an indoor cat. However sometimes I let him out when he sees the dogs in the fenced yard, and cries his face off. But the very minute I let him out, he is off to eat some grass. He will do it for about 5 minutes, and then he stops and plays a bit out there. I dont think it is harmful to him, other than that once in awhile it makes him puke (of course he waits til he is inside to do that ) But other than that he is ok. Does anyone know why cats eat grass? And can it cause them any health problems? He is a well fed cat eating hard food and soft food. Thanks.

OOps>>thought I was in a different part of the Forum. Ill ask this in the "Behavior" part! Wish I knew how to delete this!