Aftermarket headset lockrings were a solution to the problem of regular threaded headsets on mountain bikes coming loose on the trail. Before then, it was a case of tightening the locknuts together even harder together, which often caused unsightly damage/indentations to the locknuts.
The original aftermarket locknut was the Gorilla Headlock which, when threaded onto the fork steerer, could then be clamped in place via an allen key. Kona/Joe Murray took this idea a step further in 1991 by introducing the Kona Impact Headset which featured an integrated clamping locknut.
Grey Components (ZooBits)
Grey Components Threadlock
Tien Hsin Double Lock
Wheels Manufacturing Growler Headlock