That is a great question! The bungee lines reduce the "pull" on the anchor and allows it to stay set in rougher conditions. Instead of “snapping” at the anchor when waves come in, with a bungee line, the PWC just moves back a foot or so without the snapping. After the wave, the PWC comes right back to where it was originally. The waves "throw" the PWC around and this "force" is transferred right to the anchor with a normal dock line.
With a bungee line the force is substantially reduced allowing the anchor to hold in much rougher conditions. Hope that answers your question. Thank you!