The catalogs call them jointers. I do not have the right reference books with me here, but it is my understanding that they are used for compressing and smoothing the concrete in the joint. The slicking action works the water to the surface, resulting in a longer drying time and harder joint. You are basically trowelling the joint with it.
The one shown in the picture is a brick jointer. Block jointers are usually longer, often have a handle, and just have the round shape.
