From the book:

Many wargames follow a traditional pattern whereby all models are moved, then they shoot, engage in close combat and finally check morale, before going on to repeat exactly the same sequence in each subsequent turn. This linear approach works well enough for many historical periods when the emphasis fell on large groups of combatants that normally acted en masse due to common drills or fighting techniques. The Napoleonic player, for example, will be well used to dealing with entire battalions that move, shoot, reload, charge and even react to events on the battlefield as a single entity.

In addition, this will often be occurring in plain sight of an enemy that is acting in precisely the same manner. Modern combat however, tends to be less 'formal' than the preceding example, particularly when counter-insurgency operations such as those that took place in the Vietnam conflict are considered. Far from regiments standing shoulder to shoulder and acting as a single entity in the face of a similar enemy, many contacts would become significant at a platoon or even squad level. Furthermore, individual soldiers or groups might be undertaking different actions at the same time as their colleagues are performing other tasks. During a fire-fight some members of a unit might find themselves pinned down whilst others move towards cover, or attempt to gain a position of tactical advantage.

Similarly, others are pouring the maximum possible volume of suppressive fire towards the suspected location of an (as yet) unseen enemy whilst their leader is urgently requesting support from helicopter gun ships or a nearby fire base. The nature of such contacts clearly dictates that a sudden change in situation could easily result in individual combatants having to take different actions within a matter of seconds. Add to this the peculiarities of terrain, tactics, motivation and changing objectives and it becomes readily apparent that situations such as that of Vietnam made them far from typical conflicts. As such they require a less than typical system in order to recreate such action on the tabletop.