I would say in order to make more possibilities for motives. A step-father in the picture might indicate that there is tension in the family and it could give any number of people in the family a motive for murder. It could be that the step-father resents the children because they would keep him from inheriting the money (as in this case) or in another story the presence of a step-father could also give the children a motive to want to kill him. Maybe he treats the mother badly. Maybe the mother has left all the money to him and not to the children or maybe the mother trusts the step-father to manage the money for the children when she shouldn't trust him.
There are any number of variables in choosing to make a character a step-father rather than a natural father. In short in a detective story the purpose would be to offer various motives and to increase the tension. In another kind of story to maybe make the emotional relationships between characters more complex and deeper.