[PvK]

I think it's because if there are closer enemy groups which none of your forces are fighting, then your groups will go after them.

So, breaking into multiple groups with orders to attack alternate targets, and/or using Hold and Attack, or Guard Commander with the commander set to wait or do other things for 1-5 turns before attacking an alternate target, can work well. Using a commander like that to delay longer than Hold and Attack's 2 turns can be especially effective at getting a good force into the rear.

As far as effectiveness, fliers tend to land in a dispersed formation compared to men moving on foot, and most fliers (like Caelum's) are size-3 rather than size-2, which means that it becomes more important than usual the number of units in a group, because the resulting scattered fights then end up with more of your units involved per fight, which can make more difference than the strength of the fliers used (e.g. 6 Devils may be defeated in a situation where 24 light Caelian fliers with spears would win).