Excerpts from Andreas Philipp's message of 2011-01-20 08:07:23 -0500:

Hi,

Maybe it is a very stupid question but I want to ask it anyway. In
general, 'btrfs filesystem balance' takes very long to finish and
produces lots of IO. So what are the classical usage scenarios, when
it is (really) worth doing a balance?

The idea behind the balance is to spread the used space across all your
drives evenly.  So you would usually run it after adding a new drive.

It's also useful if your disk has a lot of space allocated to either
data or metadata and you want to return to a more reasonable default.
Josef has a patch to do this more dynamically that we're fixing up
(hopefully for 2.6.38-rc2).

In general, it isn't something that you want to do very often.

-chris