For the child: Your kid sees you value education. They feel proud you're there (usually—teens might feel mortified, which is also growth). You get a window into their school world.
For the school: Extra hands help. Reading with kids, helping with sports days, running canteens—these things matter.
For you: You meet other families. You understand how the school operates. You have more standing to advocate if needed.
These are real. But they're not a reason to volunteer if it stresses you or if you're not genuinely able.