ANSWER and EXPLANATION
Experimentation with the bungee cord sugested that the rule given would be a good predictor for the stretched cord length as a function of the jumper's weight w:
L = 30 + 0.2w
The operators decided to adjuct the jump off point for each jumper to L due to the rule because this height of the jump off point would be sufficient for any jumper especially after the cord is stretched.
This is because the length of the cord before the jump is shorter than after the jump (due to the stretch) and so setting the jumpoff point to be at point L would mean that they are trying to be safe having considered that after the jump, the cord would stretch to L.
That would mean that the jumper need not worry about hitting the ground after jumping.