The basic use of this weapon is to force the enemy to go to ground during an assault. This permits an assault to overrun the enemy position with less hazard.
Therefore, a SAW must be light enough for an individual soldier to carry.
A SAW can also be used to defend against a massed assault. It is less effective in this role, because it lacks the heavy water cooled barrels of heavy defensive machine guns. Many SAWs (such as the RPK74 and M249) are "stretch" versions of assault rifles. They therefore fire from closed bolts and can cook-off rounds if they get too hot. In heavy defense, a SAW may overheat.
Most SAWs in current operation are derived from only three basic patterns: Kalashnikov (Either RPK or RPK74), the Stoner Weapons System, or FN-FAL M249.
The SAW is a response to problems caused by equipping troops with a simple assault rifle. Classical assault rifles provided a machine gun function, but troops too often became excited in combat, and wasted large amounts of ammunition. Therefore, in many modern armies, military doctrine requires the average soldier to avoid using his weapon's fully automatic mode unless defending against a mass assault or an ambush.
This doctrine greatly reduces logistics loads, including combat pack weights, and aerial resupply and fuel requirements. It reduces training requirements and expense. It also extends patrol time for a typical soldier.
The problem is that this doctrine provides no fire support during an assault. The SAW was invented so that a machine gun could be carried on assaults. It is a specialist weapon to avoid unnecessary use of ammunition, and reduce both the training and combat pack loads of of a squad.
When applied to civil or irregular militia, this doctrine makes private purchase of ammunition affordable, and allows militia to train and operate with standard military doctrines using non-military repeating rifles. In war time, such lightly-equipped civil militia can be easily upgraded by distributing relatively few SAWs, one per squad, and training.
Further benefits:
Doctrine
See also weapon, assault rifle, rifle. M249 and RPK74