The first purpose that Richard's soliloquy appears to serve is to reveal the motivations and personality of the character - we can see that he hates himself because he was basically a hunchback, so he was pretty self-conscious about it. He also reveals in the first scene what he intends to do throughout the play.
As for the second purpose, I'd choose the answer - it informs the audience about the background of the plot, as it gives us reasons why Richard decided to do what he did.
