As part of the reading of Tarroka's letter, the party is informed of the location of three "treasures"; the solar sword, the sacred symbol of Ravenkind and the volume of Strahd.
Looking at the description of the Sacred Symbol of Ravenkind in Appendix C (p. 222), it seems something that a group of adventures wishing to bring down a powerful vampire would like to have. The Sword of the Sun is also obvious when you want to face a vampire; in fact, the extract from Strahd's Volume (p. 252-253) describes what Strahd fears.
But how does Strahd's Volume help the party? They really don't need to know that Strahd fears the Sword of the Sun because any vampire is right to fear any Sun Blade, and we don't need to read your diary to solve it. The adventure at least implies that it is as important as the other two elements by including them in the same Tarroka card reading that guides the party during the adventure.
However, I can't see what makes it as important as the other two elements when all it does is give it some history. It may be interesting for players to learn, but the party could happily skip the part where they collect that book (although some history would be lost) and face Strahd with only the Sword of the Sun and the Sacred Symbol of Ravenkind and still have the same odds of win.
Having the book on his person does not seem to confer any benefit to the party. It's not that every object should be a super powerful magic object, but at least I expected him to tell you a secret that would indirectly facilitate the fight against Strahd or something. I thought the idea was that if they marched directly to Strahd's castle, he would clean the floor with him, but each of these special items they acquire along the way increases their chances of success against him.
The only effect it seems to have is that it enrages Strahd, but I imagine that assaulting his castle with the other two powerful objects and trying to kill him would be enough motivation for him to be angry enough. From the description of the Strahd Volume in Appendix C (p. 221):
If Strahd sees, or finds out about a servant, that the volume has fallen into the group's possession, all of his other objectives (see chapter 1, "Into the Mists") remain on hold until the book is recovered. When Strahd attacks, his preferred target is who has the volume.
The reason I ask is because of the motivation in the character. Why would the party want this besides "because the reading of the card told us where it was, and the other two elements he told us seem quite important, so I guess this must also be …"- I don't want my players to find it, Sunsword in hand, and go"oh, it's just a story that doesn't really tell us anything useful … what was the point of that?"Keep in mind that my players are not all who make numbers, they mainly enjoy the narrative, but they still need a reason in the character to worry about this Volume.
So, is there any benefit (mechanical or narrative) that Strahd's Volume gives the party against Strahd (directly or indirectly)?