Spitz is Buck’s last steppingstone on the way to becoming Buck, Master of the Universe. He may have conquered snow, starvation, and the need to wear little dog shoes, but Buck isn’t there until he has it out with Spitz.
Because Spitz needs to go down in the end, it’s helpful that we don’t like him. He’s cruel and pugilistic (meaning he’ll start a fight with you if you accidentally breathe on him or do something minor). When he goes up against our noble and determined protagonist, we all know who’s going to win.
So Spitz’s presence is at least partly responsible for two main themes in this novel: inevitability and competition. As we mentioned, it is inevitable that 1) Spitz will fight Buck, and 2) Buck will win and take his rightful place as leader of the pack. The competition part is equally clear; this is the classic alpha dog meets alpha dog scenario.