The kinchaku is a traditional Japanese purse. It is a small bag, typically with a drawstring. In Edo Era, Kinchaku became a fashionable with rich people and high-end kinchaku were made by professional craftsmen by using leather, imported woolen cloth and silk. It was a primary accessory to carry around.
The shinobi used leather kinchaku to carry hot embers. The inside would be treated to withstand these embers. Shinobi needed fire to perform kajutsu and they developed many recipes to maintain hot embers for hours on end and the kinchaku offered a way to store these and carry them around.
According to the Giyoshu military manual, these would also be used to carry fuses. The burning end of a fuse would be placed into it when it rains or during night raids.