The purpose is to have a black balance, just as you need a white balace, ie have a non color black ie no color dominante in the blacks.
The thing is the white balance depends on the ligth you have on the set or in the location you shoot, so you use a white board to balance the highlights.
The blacks normally don't depend on the light of the set (I say "normally" because it actually is not that simple...but) So to make a black balance, you can just close the iris completly (there is position marked "C" for "capped") and make the balance.
Since then one could think of doing it once and should never need to do it again.
The thing is the components inside the camera are relativly sensitive to temperature changes, so that the black balance should be redone... sometimes...but...
Most of the new camera don't occure those changes of setup due tio temperature changes so that nowadays, manufacturers say that you nearly don't need to do black balances all the time...
You actually don't have to do it so often as before, ie aech time the light changes...
Not easy to interprete...
When you work on a multicamera, doing the black balance (manually) is something that is always done at the setup, just as the white balance.
