It could be either. Sometimes it's a heartfelt moment that leads to a happily-ever-after, while in others, it might be used as a plot device or a dream sequence. It really varies from one manga to another.
Maybe because the houseboy has qualities that are more appealing and compatible with the main character. Could be his kindness, loyalty, or a shared vision of life.
Well, it could be a character who values wealth above all else and sees marriage to a billionaire as the ultimate goal. Or maybe it's someone who initially has no interest but circumstances lead them to consider it. The possibilities are endless in the world of manga!