Looks like something to prevent the blast from going that way to me.
After that, why are they lined up like that?
The exercise target, the plate, has a couple holes in it. It’s simulating an armor/heavy barrier the marines have to get through. The exercise progresses from a couple of the marines providing cover/suppressing fire to the holes while the charge is placed. The placer returns, the unit pulls into a line to minimize blast/fragment exposure surface area and the charge is detonated.