Symbolism, Imagery, Allegory
This might not be a book about baseball—characters are off the field far more often than they're on it—but baseball plays a huge role in their lives.
First and foremost, Henry's entire life is baseball. He wouldn't be at Westish without it. Instead, he'd still living at home in South Dakota with his best friend: a baseball glove named Zero. It's also doubtful if Owen and Affenlight would have gotten together had Henry not hit Owen in the face with a baseball and brought Guert and Owen together in the hospital. Pella wouldn't have met Mike if he hadn't been depressed on the steps of the athletic center, either. So, even though Guert doesn't even like baseball—"Baseball—what a boring game! One player threw the ball, another caught it, a third held a bat. Everyone else stood around" (8.10)—it defines his life and those of the people around him, which is surprising since Westish isn't even a sports school.
On the flip side, Guert might still be alive were it not for all this stress, so maybe if he'd been more focused on making Westish environmentally friendly instead of improving the baseball team, he wouldn't have died. Just as you can't redo a baseball game, there's no way to redo these lives and find out.