Superposition is big in quantum mechanics. And actually, it’s bigger than quantum mechanics. It’s the mechanism for creating stuff out of energy. If you mix waves of different energies, you can create a thing that acts like a particle. All of our particles are built this way. Particle-ness emerges from mixing waves in the correct quantum field, of the correct energies, in the correct amounts. Since this energy is quantized, we call them energy “states”. Superposition gives us the rules about how individual states combine to form new states. While causality is the “what”, superposition is the “how”.

So every bit of stuff is built with superposition. And it isn’t just quantum mechanics, all linear systems support superposition. Any system with multiple inputs and outputs where a single input may affect multiple outputs (and/or multiple inputs may affect a single output), follows these rules. Superposition is a fundamental property of all linear systems.