As Howl and Calcifer bicker, readers get more insight into their contract and their relationship: at least when it comes to magic, they have few if any secrets from each other. And this causes tension, because Calcifer seems to want Howl to share things with him willingly, rather than just assuming Calcifer knows what’s going on. It also becomes clear that Calcifer is vulnerable in his current state, hence the plan to move him to Market Chipping.