Armies were in Warband too in the form of marshal parties.
And they aren't a substitute for the roles feasts provided:
* Benefit to immersion. It added to the feeling of a real medieval world to see lords engaging in an activity that wasn't war 24/7.
* Something for the player to do to amuse themselves during peacetime by attending feasts or doing the quest to host their own, a roleplaying feature, which adds variety and breaks up the constant cycle of map > menu > battle > map > menu > battle.
* A way of easily finding a lot of lords and ladies (both combatants and non-combatants- the latter do not join armies in Bannerlord), to get quests from or marry, during peacetime too (armies aren't meant to form in Bannerlord in peacetime, though I haven't paid attention if they properly fixed it in 1.7).
* A non-combat way of gaining relations/influence in peacetime (particularly useful if you want to do a non-combat-focused playthrough as a trader, or a smooth-talking Charm specialist backstabbing politician).
Plus, feasts would make two features already developed (musician/conversation/dancing animations, and board games) more visible to the player.