The Coping Foot is a dome shape base for the jig saw that allows you to bevel, turn and pitch the blade while changing any of those angles during the cut. Any way the blade is angled the saw is supported right at the kerf. Making a twisting cut such as you would find in a spiral handrail is a skill easy to learn with a coping foot. When the saw is held by the barrel with the blade up, very clean, accurate and complex cuts can be made such as you would encounter coping crown molding.
The Coping Foot is very compact. You can saw very close to other obstacles. Do a sink cutout up against a back splash for instance or cut a bottom plate next to a stud. Any big scribing job is much easier with a Coping Foot because you can back bevel at will without making any adjustments to an adjustment screw. Finish carpenters use the jig saw mostly for back beveling irregular lines, and so the Coping Foot becomes the ideal base for the jig saw. Many carpenters never change back to a flat base unless they need to scroll.