The piece of plastic is for insurance purposes. Primarily for charter operators or operators leading drop in dives. If a diver is certified to dive solo, they can go off diving, within their dive certification. Quarry dive sites, would also want divers to dive within their certification. One can carry redundant gear and practice using it, long before getting "trained" how to use the gear, efectively being a self reliant buddy. I have packed redundant gear from my 1st dives, and practised using it. Did not get my SDI Solo till around 170 dives; at which time, I had being diving for 11 months. So still a new diver. I got the cert, so I could dive with my buddies during my LDS drop in dives, but come up solo when my air consumption dictated.