Cale put out tue fire using Dominating water while pretending to be members of Arm so that the Empire would go on a wild goose chase trying to figure out who put out the fire they set up. And so that the Empire would not think it was the Whipper kingdom (basically Cale is protecting Toonka from Empire's retaliation) and Toonka is going along with it pretending he doesn't know it's Cale.
They also made sure to loot the castle beforehand and found magic bombs containing that alchemy inextinguishable fire mixed with dead mana. Raon explained pretty nicely what would have happened to Whipper warriors once the timer run out had Cale not come and found everything.
So that he wouldn’t be traced back to Roan.
If the Empire found out priests from Roan came to help, they’d assume Roan was supporting Whipper — and that could drag Roan into war too. Remember, the alliance between kingdoms is strictly secret.
Cale also chose the priest disguise for another reason — same logic as when he disguised himself as Arm. The Empire has the largest number of Sun God believers, so even a plain priest’s robe sows doubt. What if the Church is secretly opposing the Empire? What if these priests broke away from the official stance?
Btw, the manhwa skips a lot of this (and the translations can be rough). The novel is much easier to follow — it’s written in a very clean, easy-to-read style, and Cale’s inner thoughts make everything even more interesting.

I don't understand what's going on in this chapter