The key feature of welding neck flanges is the existence of the neck part which is designed to endure higher pressure.
When connected to the pipe, the welding neck flange is butt welded on its neck to the pipe and this transfers stress to the pipe, thereby reduces high stress concentrations at the base of the flange.
Despite the higher pressure containment capacity, the welding neck flange is relatively expensive compared with other type of flanges due to their design and engineering and increased material.