Certain lightsaber hilts look as though the were designed awkwardly. Namely, Darth Vader's and Luke Skywalker's (ROTJ) hilts have a box-like section that would make gripping it difficult.
Nearly every lightsaber hilt was unique in its construction, borne of the Force-user who created it and using whatever materials were at hand; typically, such weapons were created over a span of months. Ideally, the Jedi or Sith who created it would go into deep Meditation, poring over each individual component to be added and thus forging a connection with it through the Force.
But considering that Darth Vader and ROTJ Luke Skywalker both had robotic hands, maybe these box-like sections serve a purpose. Is this a coincidence? When Luke's robotic hand was constructed at the end of ESB, it looked like a normal human hand. I don't see what function these uncomfortable box-like piece would have in a grip of a sword-like weapon.