Soldiers often get lost in the dark. Coordinating an attack at night can be difficult. Often attacks were readied at night and then the troops would launch their attack just as the dawn began to glow red on the horizon.
Anchor the flank with a machine gun and a couple rifles. That gives a base of fire to help keep the heads of the enemy down. It is difficult to fire a bolt action rifle on the run, so having someone who can maintain fire is important.