There's a little bit of stereotyping going on in this picture, but most stereotypes contain at least an element of truth, so we're going to allow it. As any good police detective will tell you, the best way to break a suspect down in an interview is to play 'good cop, bad cop.' It works in parenting, too.
If mom is the person who does most of the nurturing and caring, and the first person that a child comes to for a cuddle when they're upset, then the child doesn't associate them with discipline or tellings-off. That can often be when dad gets involved. Mom might be the one who allows a little leeway when it comes to bedtime, teatime or bath time, but when dad raises his voice, it's time for everybody to fall in line.