In the belay system there are two things which commonly occure, and add dangerous consequenses. The first is the tendancy for the belay device (regardless of type) to creep up the spine of the biner when tension is not on the system (IE when you ar enot feeding/taking slack or when lowering/holding) and results in the potencial for the karabiner to be cross-loaded across its weakes axis & gate. The 'keeper' latch in the narrows of the Belay Karabiner prevent the biner from pivotting around your belay loop, or master-point (in top rope guide-mode)
The second scenario is an unlocked gate. This can happen either through distraction during threading the system, or the 'screw tumbler' rattles and unscrews during lengthy and repeated movements / shaking of the device. While the latter is very uncommon, an auto locking gate covers both scenarios.
