You need the rule book. But if you are playing a Wendigo character, this source book provides valuable information. Storytelling games have the emphasis placed on plot and characters. The fun comes from weaving the plot, the characters interacting with the game master's environment. It's an improvised play and the actors are the PCs.
All good actors do research, especially if their roles are outside their normal experience.
And this is where the information in the tribebooks can be invaluable. Source books add to the depth players can bring to their roles. It's like having a panel of experts pour over dry, dusty tomes in library stacks while you have a cappuccino and look over their notes. The first chapter is wholly dedicated to the history of the Wendigo, how they came from Grandmother Earth, their responsibility to protect her from the Horned Serpent and its allies. It is what a film writer would call the back story, or events leading up to the character's present.