First, taking out the wounded dwagons means they can't be added to the attack on Ansom's force (remember, they'll heal at the beginning of Stanley's turn -- if they aren't croaked first). Second, taking out the warlords prevents them from focusing their attacks selectively, presumably on Ansom himself (though we still don't know for sure how that works with warlords on both sides trying to guide the engagement in mutually exclusive ways).