Whenever my indoor cats get outside for a few minutes they head straight to the grass and start chewing. Cat's aren't exactly like dogs whereas if they head straight for the grass and start eating, they're probably not feeling good. No, cat's actually eat grass even when they feel great. Think of it more like health maintenance.
Cats naturally like to nibble on greens. According to Purina cats eat grass for the nutrients, laxative properties or to relieve an upset stomach. Providing cat grass keeps them from getting outside to get at regular grass which may have been treated with chemicals. Growing your own cat grass also keeps your cats from going after your house plants which may be toxic.
It's really easy to grow cat grass, in fact it could be the easiest plant you ever grow. You'll just need some seeds, a pot and some dirt...oh and water of course!