So, what is the pont of the battle with Calder?

[Kvess]

Calder is a wrench in gears of the Order's plans that were so very clearly laid out at the beginning of the chapter. Think like a bard: Whenever the protagonists describe a plan in detail, the narrative purpose is so we can later understand how and why the circumstances have gone horribly, horribly wrong.

The party was planning on ambushing Xykon at full strength after Xykon's crew gets weakened by a number of traps and obstacles, presumably including Calder. This fight potentially takes that option off the table. It makes the outcome of their future encounter with Xykon more uncertain. The point of the battle doesn't have to be any more complicated than that.