Two common applications for Point on constraints are shown in the figure below. Circles can be constrained to stay on a linear element by fixing the centerpoint on the line. Conversely, elements can be constrained to remain attached to the perimeter of a circle, ellipse or spline.

Note that to assign a Point on constraint to the centerpoint of a circle or arc, you must first display centerpoints (with Vertex switched on in the Creo Elements/Direct Drafting Show dialog box) and then click the centerpoint directly.