Exactly the same way Mutual Funds grow. BKMP doesn't do anything but hold shares. When the value of its holdings go up because of good business/income, the holding company grows and goes up.
BTW, that is why I could care less if there is a BKMP web site. The really important one is TFN right now.
Whatever I say is my opinion -- and I don't know anything, so don't listen to me.