No one really knows why cats eat grass. There are several theories: it settles kitty's stomach, especially if there is a hairball, and helps him feel better, it provides necessary nutrients, he loves to chew on things, grass is just there--or it is a behavioral sign.
Eating grass is not normally harmful, but you do want to watch when and how much grass your cat is eating and mention it to your vet. If your cat goes outside, you may want to be sure that he is not eating grass that is chemically treated.
If it is constant, work with your vet to identify the cause: diet or behavioral.
You can also grow your own cat grass so that you know kitty is eating safely.