Well, some things come to mind:
- Command and control for large scale operations. It takes days to pull together levies and armies in the age before top down nation army organization. But if everyone wanders off again when the troop sizes are not large enough yet, then no army will ever assemble. Sitting around and waiting for more to show up is a pretty disconcerting sensation, I would wager.
- Command and control for specific engagements. And the Battle of Maldon where the local levy breaks and runs away is a nice example of the problem the heroic ideal is trying to avoid.
This still leaves open how the tool of the heroic ideal achieves this, but that is a separate research question.