Story progression, after you gain access to the desert. You don't need to buy the Chicken Suit, save your silver for the Diablo Suit for that achievement. The chicken suit will not unlock this for you.
Also once you get all 6 orbs in the levels (7th is for beating Calaca), you can tap A (as a chicken) to fly around everywhere, basically making everything a joke. But useful for 100% each area. The only time I really wished I had it though, was the disappearing platforms for my 6th orb.