Dickens' characters often have tic-like mannerisms: Micawber, for instance, is always eager to make punch. In part, these mannerisms are simply a way to distinguish between the large cast of characters. With that said, the fact that David suggests making punch at this point in this story is significant; Micawber makes punch in a ceremonial way that underscores his status as head of his household, so David's suggestion is perhaps a way of allowing Micawber to feel empowered at a time when his professional and personal lives are in turmoil.