Position by Business Unit - Revise Positions by BU

I was informed before that you cannot change the BU in a position unless you close the old position with old BU and create/Copy the position with the new BU. But I have seen the functionality of Position by BU - revise Pos by BU which allows revision of HMCU. What is the impact of using this and updating HMCU rather than the way of closing /inactive the old and create of new.