According to Malbim, the Jews provided Achashverosh with a count of the dead in order to demonstrate to the king that the very existence of these 500 major enemies of the Jews implied the existence of countless more minor enemies.
The Yad HaMelech sees in this accounting the Jews’ report to the king of how many spoils were not touched.
The Maamar Mordechai points out that Achashverosh – having risen to the top of Persian society as a military leader – would have enjoyed hearing these numbers of military casualties and exploits.
It is the opinion of Rav Galico, however, that the fatalities were reported to Achashverosh by the enemies of the Jews in order to anger Achashverosh. It was yet another minor miracle that he did not become upset.