Alexios pointed out the benefits of valor. I figured I would chime in with morale. It is very useful to have high morale troops. Buildings that give morale bonuses such as a church and a requilary(?) are also another thing to consider building in your troop producing provences.

Having a high morale means your troops will not rout as quickly as troops with lower morale. They can take more morale reducing situations, such as being flanked or missile fire, longer. This keeps them in the field killing instead of being captured, and high morale units are more forgiving if you accidentally throw them in a tough situation. Very usefull for those that are new to the game and even some of the more experienced but less adept tacticians among us.

I think morale is the most underated stat of all the unit stats, so I often feel its necessary to chime in when I can. Think of me as a Morale Advocate.