1)
Why does the Torah find it necessary to tell us about this battle?
1.
Rashi: To teach us that Midyan and Mo'av were mortal enemies, and that is was only due to the common threat that they joined forces against Yisrael in the days of Bil'am.
2.
Ramban: To point out the tremendous strength of Hadad, King of Edom, who, when the Midyanim came to assist the Mo'avim in the Field of Mo'av, he defeated them both. 1
