It's only at 5m penalties. The defence can't move forward until the ball is tapped, but have to tackle the charger and stop him dead before he covers about 3.5 metres, from a standing start. A defender on their own is pretty much obliged to run forwards headfirst into someone big going at full belt, or concede a try.
This isn't the case with Simon's Bangers, because of the 15-20m gap that can be closed as soon as the ball comes off the top, or a tryline ruck that can be contested in a dynamic environment and requires much more skill and timing, with the defence practically at the gainline and allowed to advance as soon as the ball comes up. You rarely see it being used, tryline rucks are either pick and go or fling it wide - why might that be?
So maybe it's maintaining contest, maybe it's safety, maybe it's both.