In longer battles, reinforcements can save the day. I have had problems with overextended supply lines, especially when you're hoping to completely rout the enemy, and either you don't (and you're suddenly outnumbered in their end of the battle field) or sometimes the enemy is able to bring in reinforcements behind you (which may bring a nasty surprise).

And yes, the foot troops probably can only help you in partial retreat, or as noted, if you're mostly in a defensive position and near your reinforcement point.