That way the king got a happy mistress(now married to a favored vassal), the king could get a new mistress (and that would make him very happy), and the vassal would feel blessed to be sacking out with the king's own mistress.
Sometimes the king would give his illegitimate daughters away in similar fashion.
The relationship of Obama, Rev. Wright and Michelle suggests that such an event happened ~ and a video of prior relationships is available.
(scandalized)