PS2 Utility Disc...?

I've only heard a little bit about this elusive disc. I'm sure it allows the user to access all sorts of developer-side stuff.

But really, is there anything about it that would warrant a quest to have/get one?

(I'm stubborn. It's not so much the object, but rather the hunt that interests me.)

[hyungsup]

I heard that cd only applies for earlier versions like v0,1(the jap consoles)