Just a note about affecting big battles directly, with the right build you can still disproportionately affect the battle by targeting dangerous units like high end cav, lords, and disrupting the archer line with cav charges, instead of just carving through low end infantry, which is what the rest of the army is for.
I think killing the lords also affects how the enemy party fights, e.g. forcing the infantry to charge into the open when they should be holding position on a wooded hill.
Also, in the really big battles, you'll have command over your party, or if you are in an army, one section of the army you are in (e.g. all the infantry, one flank of cavalry, etc).