It is used to mix with the hardener when putting putty. Even if you don't have a special product, you can still use the shavings and cardboard around it, but the special product is still convenient for mixing the curing agent cleanly and removing the putty with a spatula. The downside is that it needs to be cleaned after use. If it's a piece of board or cardboard, you can just throw it away, but it must be cleaned for reuse.
(Translated by machine.)

It is very convenient because it has a small turn on the screw head and the putty of the tacker.
(Translated by machine.)