The playwright chose to show this contrast to highlight the different personalities of the Cratchits and emphasize the power of Christmas to evoke different emotions.
Mrs. Cratchit is more passionate and outspoken, readily expressing her anger and resentment towards Scrooge. Bob, on the other hand, is more forgiving and optimistic, choosing to focus on the positive aspects of the holiday and the fact that Scrooge provided them with the means to celebrate.
The contrast reveals Mrs. Cratchit's strong moral compass and unwillingness to tolerate injustice. It shows that she is not afraid to speak her mind, even if it means criticizing someone in power. Additionally, it reveals her deep love and concern for her family, as she desires a better life for them.
Bob's attitude reveals his gentle and forgiving nature. He sees the good in others, even when it is difficult to do so. He is also practical and realistic, recognizing that he needs his job and is grateful for the opportunity to provide for his family.