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The "bunching" tactic is based off of military Close Quarters Battle aka "CQB" and come from Military Operations On Urban Terrain...or "MOUT."

When conducting operations [particularly in] urban terrain, the defender has the advantage. It is their home turf. They own the terrain and control it.

As another poster mentioned...trying to take down an individual with a belt-fed weapon and body armor......imagine trying to take down a room with a sand-bagged machinegun and an individual wearing body armor.

It is what Marines in Fallujah had to face.....it isn't fun.

Patrolling in a jungle, desert, or even a city street you would want dispersion to avoid multiple casualties from a single mine, machinegun burst, etc.

However, when it comes to breaching, the entry team has to overcome the "fatal funnel." Essentially each defender is looking at a "V" toward the doorway/opening and the total of the defenders' cones of fire is an ^ shape....ie "funneled" at the opening. This essentially provides the defenders with the most optimal class of fire in respect to the target = "enfilade fire."

Enfilade fire = When the long axis of the beaten zone (where the rounds impact or potentially impact) coincides, or nearly coincides, with the long axis of the target. Simply: everywhere your rounds could hit, envelopes everywhere your target is.

An assaulting element that sent dispersed individuals through a breach would easily be picked off one at a time.

By "stacking" the assault element can "flood" the breach, their first task is to clear the "fatal funnel" (ie get through the doorway) and more or less immediately disperse once inside the room. This provides multiple targets that hopefully overwhelm the defenders, gets the maximum amount of firepower on the defender, and immediately disperses to increase the individual assaultman's chance of survival.

Stacking and breaching is the most effective means to take down a structure (short of collapsing it with a JDAM), but it can also be very bloody for the assault element. There's a reason Hue City and Al-Fallujah were such bloody battles.

At least in combat, the assault element will often have the benefit of a base of fire (that suppresses the building), and a support element that can flood more assaulters and assist with casualties.

That's the basic premise behind the "stack."