There was 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." After imprinted with false memories, Rachael was a secreatry for the Tyrell Corporation and assisted Tyrell in running operations.