The design makes D's lighter and stronger than ovals of the same material. Easier clippability than ovals. Designed for fast, easy clip-ins. Used on the rope end of a quickdraw. A metal sleeve slides over the gate to ensure that it stays closed.
Screw locking means that the sleeve screws over the gate.
These are good all purpose locking biners. An auto-locking biner has a spring loaded sleeve that locks the gate. These are a bit more expensive, but make for quicker clip-ins. These are great when you only have one free hand. Locking biners are used for all critical applications.