• The Battle for Morale
The battle for morale is waged on both sides of any given conflict. When soldiers are in dangerous or unpleasant situations, they tend to become discouraged and frightened. If they are kept in these situations for too long without a rest, they might very well run from the enemy at their next encounter._{[12¼] [13¾]}

• Keeping Pressure on the Enemy
You can only demand so much from one group of soldiers, before their morale will begin to suffer. Rotating fresh troops into battle also offers the advantage of keeping the enemy constantly engaged. Unless the opposing commander makes an adjustment, this will have negative effects on enemy morale.
 
This can also be referred to as Swarming. Military Swarming involves the use of a decentralized force against an opponent, in a manner that emphasizes mobility, communication, unit autonomy and coordination/synchronization._[13¼]

The battle of Verdun was the longest and bloodiest of WW1, with nearly a million dead or wounded between the French and Germans alone. Battlefield conditions were obvoiusly harsh, and both sides had issues with low morale. The French commander Pétain began a system of rotating his troops to Verdun, from other locations along the western front; giving him a relatively well rested defensive force. For awhile Germany countered with a similar tactic, but later had to commit those reinforcements to fight Russian forces on the eastern front. The French were able to hold, and by August 1917 were able to recapture all the territory Germany had occupied since early 1916. _{[13] [13½]}
 
In a interesting side note, Pétain was considered a hero following WWI, but got a little too chummy with Hitler, and was sentenced to death after WWII (later commuted to life).