On Sun, 12 Feb 2006 18:58:37 -0500, Guess who
wrote:

I don't know enough about these, so I have to ask. Aren't they
levelled using bubbles?

Bubbles or pendulum, depending on what you spend.

If so, what's the advantage?

Convenience. You set up one gadget in the middle of the room, then you
have a horizontal datum all the way round almost instantly. Great for
tiling.