Oh okay.
Well it goes back to what Spike says. This is from memory, so it might not be 100% correct.
"Blood, blood is life."
Ahh, fuck it. I'm looking it up.
XANDER: Why blood? Why Dawn's blood? I mean, why couldn't it be like a, a lymph ritual?
SPIKE: 'Cause it's always got to be blood.
XANDER: We're not actually discussing dinner right now.
SPIKE: Blood is life, lackbrain. Why do you think we eat it? It's what keeps you going. Makes you warm. Makes you hard. Makes you other than dead. (quietly) Course it's her blood.
If you think about it, the series has a lot to do with blood. Without it, then there would be no reason for Buffy to kill vampires since they drink it.
I'd get into it more, but I'm sleepy. But you know what I'm saying.