To allow scrubbing one's own back, bath brushes with long handles were invented. For the scrubbing of other body parts or another person, short-handled variants are better though, as they allow scrubbing with a higher and better controlled pressure. Sometimes the handle is detachable, so the brush can also be used without any handle.
Bath brushes can also be used as spanking implements, using the flat and smooth back of the wooden brush, like a bigger and heavier version of the hairbrush. Brushes with a detachable handle are usually not suitable for spanking, as the handle is typically attached to their back, which is therefore not flat and smooth. Plastic bath brushes are also generally less suitable.
Two boys bathing in a tub, drawing by Wilhelm Busch.