Analogy? I'll give it a whack... Your CD Player stereo is useless without a CD with music on it... (It's dependent on the CD). If they built CD Players with the CD already in it, it would get boring very quickly...
So they build them so you can "inject" the CD, (on which it is dependent) into the player. That way you can inject a different one each time, and get "different" behavior (music) dependent on which one you inject.
The only requirement is that the CD must be compatible with the interface defined by the player. (You can't play a blue-ray disk in a 1992 CD player.)
