This one is practically a no-brainer – of course blood is important in a vampire book. But what, exactly, do all the references to blood mean? Renfield is the only character to really explain it, and he does so in fairly reasonable tones to Mina:
"I tried to kill him for the purpose of strengthening my vital powers by the assimilation with my own body of his life through the medium of his blood – relying, of course, upon the Scriptural phrase, 'For the blood is the life.'" (18.16)
So, according to Renfield, anyway, to consume someone else's blood is to consume some vital part of his or her life. If you consume enough of their blood, you gain their "vital powers" (and, obviously, they die).
Of course, we have to take Renfield's explanation with a grain of salt, since after all, he's locked up in an insane asylum for a reason. But his explanation does make a certain amount of sense, when you compare it to what Dracula is up to – he appears to have gotten younger after moving to England and feeding on Lucy. She becomes weaker as he becomes stronger. Perhaps Renfield is right – maybe in the world of Dracula, consuming someone else's blood really does allow you to "assimilate" some of their "vital powers."