Preparations allow adequate penetration or improve overall coverage. An edge joint can be welded on all sides, providing additional joint rigidity, but they could require post-weld grinding if the weld must be flush with the base material.
Remember, the primary forces causing stresses in welded joints are tensile, compression, bending, torsion, and shear. Edge joints, especially square groove joints, are notoriously weak since the weld does not fully penetrate the joint thickness.
Also, edge joints are more susceptible to corrosion and defects
such as porosity, lack of fusion, and slag inclusion because of the overlapping parts.