Some perks for captain, like pathfinding (+movement speed) on global map, better looting chances, diplomacy, mentor for brothers, scouting and other small but nice bonuses to global map, events, maybe inbattle modifiers/
XP = XP total earned / 100
Or, for siplification, captain earns perks for ranks in reputation, or completing ambitions.
It will helps captain figure to be not so formal.