Tsukamaki is the art of making, maintaining, and wrapping sword handles.  Handles will often need work for two main reasons: first, they receive a great deal of friction, torque, sweat and oil from your hands, and other types of abuse as you use your sword in practice.  Second, it's a place where most manufacturers compromise by using substandard materials and inexpert labor.  If you want to improve your sword's feel and performance, upgrade your handle!