Well, it's meant more as 'warfare' than 'nation building' as even if your noble you at best can expand or build more your influence or your personal stronghold fancier. Not run politics or lead nations. Nobles and commoners alike are vassals to their nations, nobles just got more to deal with as they use armies build from their followers to organize them according what supreme commanders/Coronals ask them lead to (and what to achieve).
But besides warfare there is spying, espionage and court room balls between nations who are not directly in war (or may be allies to nation you hail from). So if you sense your nation is going to be attacked you may want secure allies to your nation's side when things come to brunt hit.