If you are bending particularly curly wood, or wood with excessive run-out of the grain, the metal strap will support the outer surface to prevent grain separations. This is especially useful if you are making tight curves as those in a cutaway.
Mostly I don't use this but have on occasion. I do all of my bending over a hot iron pipe and have not used a bending machine.
Seems to me that they limit you in that any shapes that you want to create will need to be fixtured.
I would recommend that you develop your skills in the art of hand bending.