You always try to cover an obstacle with fire, so the idea is while they're milling around slowly piling up soil they're vulnerable to fire. You can see in some of the photos the troops looking down on the obstacle from I presume potential fire positions.

> You always try to cover an obstacle with fire,

I misread your comment to mean flames/burning stuff (not weapons fire), and wanted to pause for a moment to admire your approach to life.

:)