My dad had a set of those, and I used them often. Come to think of it, I probably still have them. They are used to keep the two rubber cups and the internal gear inside of a drum-brake wheel cylinder, while the brake shoes are removed. In my experience, you rarely needed to use these clamps except when you had just rebuilt the cylinder, and had installed all new spring, cups, boots, etc. Then since the inside cylinder surface was now shiny and smooth, and brake lube or brake fluid had been wiped on the new parts, there was a very real risk that everything would just pop out and land on the floor.