Also keep in mind that in many cases battles last more then a few hours even in the modern era. Way back when battles often lasted for days or weeks, so you could have a combat phase wherein during the day you are attempting to make a push, and a night phase where you might rest up/and/or try and sneak into the enemy camp and slay a particular target or steal some plans. You might also have days where there is no heavy action and its just the occasional bowman taking pot shots, you might again try to sneak past enemy lines and try to break their supply lines so that they run out of food and their army starts counting as fatigued. From a setting perspective a battlefield offers you a lot of different sort of objectives.