God-Emperor
Deity
I had a portrait of the organizer for Israel pop up,
and once I had control of him, I selected the yellow flag upgrade
to turn him into a leader. When I moussed over him afterwards,
he had 5 "uses" left. He did start out as a great general. Is there some
place I can find more information about this?
There isn't much to know. It has pretty much been mentioned already, but here it is in detail:
The commander (Great Commander, or Field Commander) unit has control points, which start at 5. He also has a command range, which starts at 1. Every time a unit of yours within his command range attacks another unit and wins he gets XP, up to a maximum number of units per turn equal to his control points. Likewise he gets XP when the nearby units win while defending. Using those XP he can get promotions. These promotions have a special property: they apply to every unit within his command range. So that first promotion he gets, Morale I (as I recall, there is no other choice, this is the only one initially available), gives +5% combat strength to every unit within the command radius (which is 1, so all units int he same plot or in an adjacent plot). More or less, since it only applies to the first 5 (the default number of control points, it can be increased by promotion later) attacks done in a turn. I think it applies to all defensive combats without limit during your enemies' turns but I am not sure of that.
So it is basically earning extra promotions which are applied to nearby units, making them stronger. This increases the rate at which your units get stronger from promotions since they both get their own promotions' effects and the effects from the commander's promotions all from the actions that get your units some XP. The effect can be quite large - that first +5% strength promotion is probably the weakest one until you get to the ones that add control points and things like that, for example the Morale 2 promotions is +10% combat strength and Tactics I gives a +15% withdrawal chance. If the commander has all 5 Morale promotions it gives +50% strength and some other benefits.
The number of XP that the commander needs to get promotions appears to be 50% more than a normal unit needs, but they add up pretty quickly since they can get XP from multiple combats in a turn.