Motivation is the reason your characters do the things they do. Characters need to have a solid motivation for all the decisions they make throughout the story, but…the most important motivation you need to give them is the motivation to participate in the story in the first place. Motivation is the reason the stakes matter.
[Harry Potter’s] goal is to destroy Voldemort.
Voldemort will take over the wizarding world otherwise—this is what’s at stake.
But why does Harry want to stop Voldemort himself? Why not step back and let someone else do it? Because Voldemort killed Harry’s parents. That’s Harry’s motivation.
Motivation is the emotional investment in the story for both the character and the reader. It gives the characters depth, which makes them more engaging. The reader wouldn’t be very interested if, at any point, the protagonist could decide that the stakes aren’t such a big deal after all and go home. Motivation locks the character into the story.