Funny, I had the opposite question, "I've read about forage cakes, but where do I buy them?" I wondered if anyone had tried them.
I read about forage cakes in one of my chicken books ("How To Raise Chickens," Christine Henrichs). It has a FULL PAGE devoted to singing the praises of forage cakes. Seriously, I triple-checked the article looking for "paid advertisement."
"Forage Cakes are essentially giant granola bars, formulated as nutritional/behavioral supplements for chickens and other fowl. Used as directed, Forage Cakes will help captive birds better utilize their entire diet."
They are made with a higher quality set of ingredients compared to regular chicken feed, so they will digest better - so that the chickens need less food. They're also less messy, and give the birds something to peck. "Embedded treats such as nuts, seeds, grains and flower petals encourage natural foraging behaviors, which is highly stimulating for confined chickens."
My guess is that the forage cakes would be awesome for battery/commercial hen houses. But surely our backyard chickens get plenty of nutrition, treats, and entertainment. And without having run the numbers, I'm going to guess that forage cakes cost at least 10x more than regular chicken feed.