In Curse of Strahd, as part of the Tarokka card reading, the party are told the locations of three "treasures"; the Sunsword, the Holy Symbol of Ravenkind, and the Tome of Strahd.
Looking at the Holy Symbol of Ravenkind's description in Appendix C (p. 222), it looks like something that a group of adventures wanting to take down a powerful vampire would want to have. The Sunsword is also an obvious thing to want when going up against a vampire; in fact, the excerpt from the Tome of Strahd (p. 252-253) describes how it's something that Strahd fears.
But in what way does the Tome of Strahd help the party? They don't really need to know that Strahd fears the Sunsword because any vampire is right to fear any sun blade, and we don't need to read their diary to figure that out. The adventure at least implies that it's as important as the other two items by including them in the same Tarokka card reading that guides the party throughout the adventure.
However, I can't see what makes it as important as the other two items when all it does is give you a bit of history. It might be interesting for the players to learn about, but the party could quite happily skip the part where they collect that book (although they'd miss out on some background lore) and take Strahd on with only the Sunsword and the Holy Symbol of Ravenkind and still have the same odds of winning.
Having the book on their person doesn't seem to confer any benefits to the party whatsoever. Not that every item needs to be some super-powered magic item, but I was at least expecting it to tell you a secret that would indirectly make the fight against Strahd easier or something like that. I thought the idea was that if they marched straight up to Strahd's castle at the start of the adventure, he'd wipe the floor with them, but each of these special items they acquire along the way increases their chances of success against him, but it seems that having the Tome doesn't affect their chances whatsoever.
The only effect it appears to have is that it makes Strahd angry, but I imagine storming his castle with the other two powerful items and trying to kill him would be enough motivation for him to be angry enough already. From the Tome of Strahd's description in Appendix C (p. 221):
If Strahd sees, or learns from a minion, that the tome has fallen into the party's possession, all of his other objectives (see chapter 1, "Into the Mists") are put on hold until the book is recovered. When Strahd attacks, his preferred target is whoever has the tome.
The reason I ask is because of in-character motivation. Why would the party want this thing besides "because the card reading told us where it was, and the other two items it told us about seem pretty important so I guess this must be as well…" – I don't want my players to find it, Sunsword in hand, and go "oh, it's just some lore that doesn't actually tell us anything useful at all… what was the point of that?" Note that my players aren't all number-crunchers, they do enjoy the narrative primarily, but they still need an in-character reason to care about this Tome.
So, is there any benefit (mechanical or narrative) that the Tome of Strahd gives the party against Strahd (directly or indirectly)?
Best Answer
It reveals some key backstory that makes the PC's journey much easier.
It helps with certain encounters involving Sergei and Ireena.
p156 notes this.
The PCs have no obvious way of learning that Sergei and Ireena should be brought together outside of the book. Knowing what the book says makes it much more obvious what is up and lets them avoid bad encounters.
It helps with decoding a letter about the abbot.
It helps you learn this from p114 which helps you rally locals against the abbot and his efforts.
It influences Strahd.
He himself is really worried about the PCs learning his past was a talkative man who made a pact with a dark power rather than just a vampire god, with an end result that he will fanatically pursue the PCs or whoever they give the book to. This lets them lure him into an ambush.
Notably his preferred method of attack is an excellent one to encourage PCs to use the information within. p6 explains this.
He spies on them, invites them to come to his castle, and seeks to turn them against each other. He may well seek to induce several PCs to kill their companion for the sake of the book or otherwise reveal key details. His method of attack isn't to directly attack the PCs- they will normally have time to layer on buffs and protections if they can resists his temptations.
He also explicitly likes to try to turn PCs against each other. If he wants to kill one player his desire to kill that player will be obvious as he will incite people to slay them so he may take the diary back. This is only obvious after taking the diary.
It's also exploitable - all the buffing/healing/resistance magic can be piled on the PC that Strahd will target allowing the rest of the party to act without worrying about being targets.
It levels you up.
The PCs gain a level when they find it. A level is helpful.
The benefits are rather lackluster, but there are the benefits. I personally make it a magical diary like Tom Riddle's diary and give it useful commentary and information on events around the characters from his perspective.