In some applications and on some projects, an effective way of joining leather can be via gluing. In order to help ensure a strong bond between surfaces, glues generally benefit from having a rough area on which to form their bond.
When finished leathers are mostly smooth, a hand leather rougher tool digs into and scratches up the leather, creating a rough surface. This newly-roughed surface will greatly help the adhesive set into and join the leather pieces once dried. Hand roughers are relatively common in saddle making and related work.