There are other things a Major Units can do. Major Units with a combat bonus give that bonus to every Minor Unit in the attack.  This is a good way to turn a large stack of fair to poor troops into killers. So if you've got a large stack of units that attack with 10 attack points, and they're stacked with a Hero with a Combat Bonus of 15, they ALL effectively have an attack of 25! This is in addition to the fact that the top three units will have tripled attack.  This is particularly effective because  generally speaking, your poor troops will be proportionally less expensive point for point.  

Heroes also accumulate experience points.  Experience Points can be used to increase the Hero's Melee Attack,  Combat Bonus, or Magic Attack.  Heroes get experience points either by being victorious in combat or by growing up (See OffSpring).