Bones-- What's the difference between Eyes and Irises?

Concerning Portraits

I cannot for the life of me figure out the difference between the two bones for Eyes and Irises.

Don't they control the same functions? I mean, it's the eye lids that close the eyes, and the Irises that actually move the eyes. So what good is a bone for "eyes"? Is it just there as a stationary anchor for the Irises and EyeLids bone animation?

it may be a helper to transform the eyelids & irisis, think of it as bone_root. I'm not sure of it, because I never make portraits, I despise them.