Shakespeare uses it as a symbol of Richard's deformity of soul and as a signal of his villainous nature and depravity. There are many cruel depictions of Richard's hump in the play. Even he himself says, at the beginning of the play, that he was born that way, and born prematurely. Being repulsive as he is, it is hard for him to find any pleasure in life. This is the initial, psychological motivation for all of his misdeeds.
It's a use of literary styles and techniques to create factually accurate narritives. I found this on google.
