There is a problem with his more recent, advanced Nexus-6 Replicants - they start to develop their own emotions. The other Replicants we see provide plenty of evidence that this is true. They then start to rebel against being slaves. Tyrell believes that this is because they have no framework within which to deal with their new emotions. Thus, if he can just gift them with memories, he hopes that this will help them cope better and therefore be a "better product."
Rachael is created, but more than just having implanted memories of Tyrell's niece, she is led to believe that she is human. We do not know how long she has been "switched on", but Tyrell admits that he thinks she was "beginning to suspect."
Involvement with DeckardEdit
Rachael figures it out and is then ignored by Tyrell. In desperation she turns to Deckard, who has been told by Captain Bryant to terminate her. But he has fallen for her. Many possibilities exist in interpreting their relationship, some of them depending on whether or not you believe Deckard is himself a Replicant.
Both of them are allowed to live - Deckard by Roy, and Rachael by Gaff, who leaves his calling card (foil unicorn) to show he's been there. Deckard and Rachael leave together. In the original theatrical edition of the film, Deckard's voice over as they drive north suggests that she may have a longer lifespan than any of the other Nexus-6s that were created.