Why was the identity of the Half-Blood Prince important to the story?

Question

Severus Snape was half-blood (as his father was a Muggle).

He referred to himself as the Half-Blood Prince.

The entire plot of the sixth year revolves around the Half-Blood Prince. My question is: why was this relevant to the plot?

Answer 1

Long-winded analysis follows arguing the toss, but there is one way in which it is very important.

The fact that Snape was the Half-Blood Prince enables him to identify that Harry cursed Malfoy with Sectumsempra. Hence he asks about Harry's Potions book and bids Harry to bring him his books. This is what causes Harry to hide his copy in the Room of Requirement. Which is how he first comes upon the Ravenclaw's Diadem Horcrux, which, of course, is how he knew where to find it in 7.

Yes it is important to the story in and of itself.

However:

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Well, it's not strictly necessary to get us from Harry surviving the Killing Curse in book 1 to Harry defeating Voldemort in book 7.

However, the character of Severus Snape is one of the most important and interesting in the series (including to the plot, for he is, of course, heavily involved in Dumbledore's machinations). Obviously, in The Philosopher's Stone, you think he's the villain for most of it; then it turns out that really he was on Harry's side all along. After that, there are a number of moments where his allegiance is questioned, but the facts of The Philosopher's Stone more or less keep you, Hermione - and even Harry and Ron - satisfied. Then, in The Goblet of Fire, you learn he was a Death Eater and then, of course, comes Spinner's End - which leaves quite the question mark. I dunno about you, but I fell for it, believing him to be Voldemort's double-agent. Then he kills Dumbledore and it seems confirmed. You despise him.

This of course sets us up for the final, great twist in The Prince's Tale, my all-time favourite chapter.

Snape being the Half-Blood Prince has a part to play in all of this. Here you have this book

the book that had taught Harry so much ... the book that had become a kind of guide and friend?

Harry Potter and the Half-Blood Prince - p.491 - Bloomsbury - Chapter 24, Sectumsempra

This book that is a mine of information:

Harry woke early on the morning of the trip, which was proving stormy, and whiled away the time until breakfast by reading his copy of Advanced Potion-Making. He did not usually lie in bed reading his textbooks; that sort of behaviour, as Ron rightly said, was indecent in anybody except Hermione, who was simply weird that way. Harry felt, however, that the Half-Blood Prince's copy of Advanced Potion-Making hardly qualified as a textbook. The more Harry pored over the book, the more he realised how much was in there, not only the handy hints and short cuts on potions that were earning him such a glowing reputation with Slughorn, but also the imaginative little jinxes and hexes scribbled in the margins which Harry was sure, judging by the crossings-out and revisions, the Prince had invented himself.

Harry Potter and the Half-Blood Prince - pp.223-4 - Bloomsbury - Chapter 12, Silver and Opals

This book without which:

I'd never have won the Felix Felicis. I'd never have known how to save Ron from poisoning,

Harry Potter and the Half-Blood Prince - p.496 - Bloomsbury - Chapter 24, Sectumsempra

And you and Harry grow very attached to it, defending it against Hermione. You're taken in, just like Harry:

He broke off, looking out of the window. He could not stop himself dwelling upon Dumbledore's inexcusable trust in Snape ... but as Hermione had just inadvertently reminded him, he, Harry, had been taken in just the same ... in spite of the increasing nastiness of those scribbled spells, he had refused to believe ill of the boy who had been so clever, who had helped him so much ...

Harry Potter and the Half-Blood Prince - p.594 - Bloomsbury - Chapter 30, The White Tomb

This adds to your sense of betrayal, it contributes to your feelings about Snape, it is all part of the backstory of Snape, with its many twists and turns, which all contribute to the power and clarity of one of the great secrets of Harry Potter. Hated Snape, Dumbledore's murderer, being perhaps the bravest man, sacrificing himself, working for Dumbledore all along. And why? For his one great love.

I think it matters, I think it matters that we learn that our friend the Half-Blood Prince was the man we now despise, Severus Snape, at the end of book 6.

But, more than that, it's also a nice little look at trust, we get taken for a bit of a ride, or seem to anyway. And, at the end of 6, it has the purpose hinted at above of excusing Dumbledore's trust. We might be too tempted to wonder how the great man could have been hoodwinked by Snape, had we not ourselves been.

But, in some ways, this is overanalysing. Why shouldn't it be simply character development? Simply the development of one of the most crucial, controversial, and courageous characters in the series?

I mean what's special about Harry Potter? It's Hogwarts, and Quidditch, and the House Cup, and Honeydukes sweet shop, and Potions, and Transfiguration, and Charms, and dinner in the Great Hall, and Gryffindor Tower. It's the everyday stuff, it's the relationships you build, over literally thousands of pages, with these characters that make you feel so thoroughly immersed in the world. That make rereading the books like seeing old friends. Strictly speaking almost none of that really advances the plot, but I couldn't be without it.

And finally, it's strange. The Half-Blood Prince's book is pivotal to the story. It's how Harry gets Felix Felicis, which is how he gets the memory from Slughorn. It's also how he learns about bezoars and it's a great bit of magic for us.

But the title doesn't just refer to the book. There's a double-meaning. Harry Potter and the Half-Blood Prince isn't just about Advanced Potion-Making, it's about Harry Potter and Severus Snape. Snape is obviously a huge part of this book,

killing Dumbledore.

We spend the whole plot wondering about him and Malfoy and what they're up to and whether Snape really is a Death Eater, and who Malfoy's trying to kill and so on and so forth.

The fact that Snape = The Half-Blood Prince, however, is not a major detail as far as pure plot advancement really goes. And yet the premise of your question seems to be that it is important to the story, but you seem to want us to tell you why :P I mean, you decide how important you think it is.

Answer 2

As others have said, it's about the plot driven by characters not about plot driven by action. The resolution of the half blood prince identity put ourselves in the Snape's youth position and creates a bond between the arch-nemesis of Harry Potter in Hogwarts - Snape - and our little hero (by the way Voldemort I think, is Dumbledore's Nemesis - at least at the time of this book)

From the first page you can feel that this book is different, special. It's the first in which the first chapter is not about Harry and his famous warm-days of summer, with The Unbreakable Vow the rhythm we are so familiar with, changes acquiring a darker tone that stays for the rest of Harry's story.

(Personally it's my favorite HP book.)