Knight instead of queen offers different moves. Bishop instead of queen offers fewer moves. It's easy to imagine (and demonstrate, as you have) positions where a promotion to knight is advantageous, but it's hard to imagine positions where a promotion to bishop is advantageous except that it may confuse the opponent (computer or otherwise).

Well, on a little bit further reflection, possibly you may want to promote to bishop instead of queen if it helped you get to stalemate when you were confident you wouldn't win. Or as others said, to avoid the opponent having an opportunity to get stalemate.