The whole thing where he came from. It doesn't add anything to the character. It doesn't make him anymore menacing in the OT. In fact, it makes him more stupid... he goes against a Jedi and dies the same stupid way his father did.
And how does witnessing the death of his father relate to the fact that he becomes a bounty hunter? And not even (from an OT POV) as a Jedi Killer. So it's obsolete, it doesn't make the story move on, in the end it doesn't explain anything, it's just a pretentious, unwise and completely unnecessary attempt to add depth to a character that doesn't need that 'depth' to be cool. He always was cool, before they changed his voice into a kiwi one.