They work well for holding your rail on a vertical surface like a wall, or holding your rail on a cutting surface that might be hard to clamp in place for one reason or another. I tried them out at the Festool class and they seem to work pretty well. From looking in the catalog I never thought about buying them, but after trying them out, I plan to get a set.
Y'all forgot the most important aspect: SUCTION CUPS
You'd use it on granite, melamine, plastic, etc that is non-porous and to which a suction cup could hold.
Would not work so well on regular wood, plywood, etc.