Add surface curve adds another surface curve to the surface you selected with the mouse or defined by coordinates. This is often a preliminary step for a subsequent modify operation based on the new surface curve if there wasn’t a surface curve or link where you needed one.
This edit breaks the link between the surface and the method originally used to construct it. You can’t open the surface’s properties dialog box now and modify it based on the parameters that originally defined it as those parameters no longer apply directly to this surface.