DECKARD: You ever tell anyone that? Your mother, Tyrell? They're implants. Those aren't your memories, they're somebody else's. They're Tyrell's niece's. Okay, bad joke, I'm sorry... No, really, I made a bad joke. Go home, you're not a Replicant... (sigh) You wanna drink? I'll get you a drink.
Deckard tells Rachael the truth—before feeling bad and claiming that he made it up. Memories form the bedrock of identity in Blade Runner, and Rachael's entire world is founded on this fictitious bedrock.
DECKARD: Tyrell really did a job on Rachael. Right down to a snapshot of a mother she never had, a daughter she never was. Replicants weren't supposed to have feelings. Neither were blade runners. What the hell was happening to me? Leon's pictures had to be as phony as Rachael's. I didn't know why a replicant would collect photos. Maybe they were like Rachael. They needed memories.
This quote is from the voiceover—which isn't included in every cut of the movie. Here, Deckard is musing on why the replicants need memories and are collecting photos—it's like they're trying to create identities in the short amount of time they still have, thereby treasuring their lives even more.
TYRELL: After all, they are emotionally inexperienced with only a few years in which to store up the experiences which you and I take for granted. If we gift them with a past... we create a cushion or pillow for their emotions and consequently we can control them better.
DECKARD: Memories. You're talking about memories.
Tyrell thinks about memories entirely as a means of control. He's indifferent to the fact that he's messing with the replicants' whole sense of reality—while at the same time giving them a basis for their human feelings.
RACHAEL: I didn't know if I could play. I remember lessons. I don't know if it's me or Tyrell's niece.
Rachael's world is in tumult after Deckard tells her that she's a replicant. She's unsure whether the things she knows about herself are really things about her or about someone else. If her entire sense of self comes from Tyrell's niece's memories, it would almost be like she doesn't have any sense of self that she can genuinely call her own.
ROY: I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I've watched c-beams glitter in the dark near the Tannhäuser Gate. All those... moments will be lost in time, like tears...in rain. Time to die.
When Roy remembers the things that have happened to him in his life, he apparently isn't remembering any false or implanted memories. He's remembering his actual experiences as a soldier, grounding himself firmly in his true identity. Poignantly, he imagines death sweeping away the memories that make up that old identity as if they were "tears in rain." These memories die with him.