When you first create a spline there are options to create it with or without the "Control Points". These are points that can be used to control the shape of the spline. While you can use Drag Control Point in this function without having the Control Points actually defined, having them enables you to use them in Dimensions, Constraints, and in the normal Drag function. This option allows you to create them, or any that may be missing due to deletions or insertions of Fit Points since the original spline definition. Any time you insert a Fit Point, a Control Point "location" is also added, but this option is needed if you want an actual point created at that location. Also note that when you use the Delete Fit Point option, it will remove a fit point location, and the point if it is defined there.