Why do I want to clamp my joint line?
You want to clamp your joint line to ensure a flush joint. Because your screw enters at an angle, pieces will naturally try to shift. With the clamp firmly and correctly positioned on the joint line, you minimize the potential for "workpiece shift" and create the strongest joint possible.
For best results, always place the large face of your Kreg Face Clamp on the "show side" of your joint. The "show side" is the side that will show on your finished project.