The Weld Neck Flange is normally referred to as the "high hub" flange. It is designed to transfer stresses to the pipe, thereby reducing high stress concentrations at the base of the flange. The Weld Neck flange is the best-designed butt-welded flange of those currently available because of its inherent structural value. It is expensive because of the design.
These are mainly used on heat coupling of pipes when there is possibility of expansion of the section while bending stress on the coupling.