This tool was made for the mechanics back in the days when you rebuilt the W/cylinders on the car. It's not too easy to pop all the pistons out so this tool idea came up.
Back when these cars we like were new the owners and mechanics didn't spent days doing /painting the brakes.
They had to get em done in less than a day and I suppose this tool speeded things up a bit to make more profit and make it easier for the mechanic to get his job done faster.
I use a pair of snap ring pliers to remove unstuck pistons...air or grease to push out stuck pistons.
I now always re-sleeve now days.---even new replacement cylinders foe the elimination of corrosion W/DOT 3 or 4 and Silicone 5 too.