Fire at all levels by artillery, mortars, and machine-guns is exploited to hold adversary attention and pin him down hence—
Fire together with gas and smoke (as well as fog and mist) represent an immediate and ominous threat to capture adversary attention, force heads down and dramatically obscure view, thereby cloak infiltrators movements.
Dispersed and irregular character of moving swarms (as opposed to well defined line abreast formations) permit infiltrators to blend against irregular and changing terrain features as they push forward.
Taken together, the captured attention, the obscured view, and the indistinct character of moving dispersed/irregular swarms deny adversary the opportunity to picture what is taking place.
Result
Infiltration teams appear to suddenly loom-up out of nowhere to blow thru, around, and behind disoriented defenders.