The Spencers are an old aristocratic family with associations to royalty for centuries. Their line can be traced to James II and Charles I, and all the way back to William the Conqueror. Diana's family owned property next to the Queen's palace in Sandringham and as a girl she played with Princes Andrew and Edward. Diana's father was equerry to both QEII and her father, and her maternal grandmother (Lady Fermoy) was lady in waiting and close friend of the Queen Mother. It's said that Lady Fermoy and the Queen Mother were responsible for getting Charles and Diana together and encouraging the marriage. As far as the royal family was concerned, Diana had the perfect pedigree to become the wife of the heir.As for Diana bringing in new blood....same as with any bride marrying into the family, really. It's not like she was a "commoner" like Kate.
It's William's wife, Kate, who as a commoner is bringing new blood to the royal family.
Harry may have been the same person he is today if his mother had lived, but I do wonder if her influence could have made a difference.