Imagine you want to drop a load of coins on the ground in a top-down, 2d world using 2d physics. How would you do it?
Attached is a capx showing several attempts I've made to bounce an object a set amount of times on an arbitrary Y coordinate.
The advantage to setting Y coordinates as collision axis is that you can create 5 coins that randomize where this Y collision will be and each bounces in a unique place. If you tried to do this with invisible immovable physics objects, you would not get the desired FX.
Tips: try increasing and decreasing the bounceStrength instance variable of the green physics ball. The blue ball is just a graphic that is pinned. In a real game you'd probably make the physics object invisible.