It's hard to ram under things that are attached to the formwork.
This device allows us to plumb the formwork by adjusting the angle of the brace.
Once sand and gravel start flying they can work behind the waler board, obstructing firm, even pressure and effecting the finish of the wall. Some masking tape along the boards can help keep the mix out.
That's a strongback with wedges keeping those walers tightly in place.
Hope this helps.