It was intended to either cause the HEAT round to detonate before it hit the vehicle's main armor or to jam/disrupt the fuse completely. However, like Schürzen (originally developed to counteract Soviet ATR), they could sometimes have the unintended effect of providing better standoff for the HEAT round and actually increase its effectiveness.
Everybody tried to uparmor their AFVs against panzerfausts. The Russians used bedsprings, the British used spare bogie wheels
and the Americans tried everything they could. Very limited success overall.
The Marines on Iwo Jima used wooden frames over their M4s to keep Japanese magnetic charges from adhering to the hull.
Also used to weld long nails [points up] around the hatches to discourage suicidal boarders.