The usefulness of the standing wave idea

Anything mechanical that can vibrate and has edges may have a standing wave on it. So the shaft driving a ship's propellers, or turning a turbine, can go into a standing wave oscillation, flexing as it turns. The wings of an aircraft also flex like a springy ruler.

Two-dimensional standing waves are likely wherever flat panels can vibrate, so they matter to the motor and to the building engineer. Three-dimensional standing waves are a problem for acoustic engineers. A good example is a loudspeaker cabinet enclosing a volume of vibrating air.

Electromagnetic waves too can produce standing waves. Radio waves can form standing waves inside metal cavities. Radio waves have been used both to make very accurate measurements of the velocity of the waves, and in the design of powerful high frequency generators of microwaves.