Hehehe, good! That's pretty much exactly it. You're going to be spinning the piece as you work on it, and if it's off-center, its momentum is going to push it out from the center of the wheel and it will be lopsided. If it's on the center, you won't see that happen; it's pushed out equally at every point, so your piece will be symmetrical.

Now!

[Hange picks up a near lump of clay, lifts it up over her head, and whams it down on the center of the wheel :D]