Your maximum energy is increased for every six cages collected. Note: There are a total of 60 cages in the game.
Rayman 2 reference
In the Hoodlum's factory, get to the place where you have to jump across crates to get to the other side. Once there, keep going up. You will eventually reach a location with a switch to activate the lifts to get to this second "conveyor belt". On those lifts, get pretty well up to the top. There is a hole in the side of a huge "pipe". Jump and helicopter over there and go in. You will see some of the villains from Rayman 2, including Admiral Razorbeard and his other cronies.