Three times in the chronicles of Narnia, Lewis describes a character cleaning or drying his sword. In none of them is it necessary to the rest of the plot.
The Lion, The Witch and the Wardrobe:
Peter, still out of breath, turned and saw Aslan close at hand.
"You have forgotten to clean your sword," said Aslan.
It was true. Peter blushed when he looked at the bright blade and saw it all smeared with the Wolf's hair and blood. He stooped down and wiped it quite clean on the grass, and then wiped it quite dry on his coat.
"Hand it to me and kneel, Son of Adam," said Aslan. And when Peter had done so he struck him with the flat of the blade and said, "Rise up, Sir Peter Wolf's-Bane. And, whatever happens, never forget to wipe your sword."
The Last Battle:
The King was still so angry that he hardly noticed the cold of the water. But of course he dried his sword very carefully on the shoulder of his cloak, which was the only dry part of him, as soon as they came to shore.
Also The Last Battle:
Then he inspected Eustace's sword and found that Eustace had put it back in the sheath all messy from killing the Calormene. He was scolded for that and made to clean and polish it.
(All the quotations are from copies I found on the Web and may be somewhat inaccurate.)
Why did Lewis include these? Why the obsession with this topic? To the extent, even, that Aslan — the Jesus-like character — attaches such importance to it in the first thing he says after one of the battles. Yes, of course, characters should clean and dry their swords, but I don't recall other books stressing it so, nor do the Narnia books stress other things that are part of every battle or everyday life (for example, I don't recall that any character ever had to use the restroom).
(Of course, many aspects of the Narnia stories are an analogy to concepts in Christianity; perhaps this is one such? Does Christianity attach some especial value to clean, dry swords, or to something analogous?)