You can have way more complicated formations in mods, like shield wall, pike wall (cavalry will die if charging this formation), archers' loose formation, etc. In vanilla Warband you may change one variable in module.ini and have "advanced" command system from Napoleonic Wars, allowing you to perform some advanced moves.
Just as Ross Ridge said, once your troops engage in melee, formation stops being very important. But it still is while they are engaging, because it's important how many of your and enemy's troops engage at once. It is also very important if you command lots, lots of troops, which may happen in Warband singleplayer if you have a good machine and set battle size to very high values. For example, if your enemy has a 3-row line of 45 soldiers and you flank it with your 5-row "line" of 30 soldiers, your enemy will get steamrolled kinda quickly, while just charging them form the front or from back may get your own troops killed.