A .38spl hull gets thicker towards the base.When you seat a hbwc,which is much longer than a swc or rn,the thicker brass tends to push inwards the thin fragile skirts of the hbwc as the bullet gets seated deep inside.
The lower canelure tends to relieve some angle of the brass between the 2 cannelures so that the thicker brass will not deform the skirt of the bullet while being seated in.
As far as I know,tests have never been ran so as to proof the validity of such.