Why does the processing of light and shade often appear in photography?

The processing of light and shade is often expressed as "tone" in photographic works. Generally speaking, photos that are mainly white and light gray that are close to white are "high-key", photos that are mainly black and dark gray that are close to black are "low-key", and those in between are mid-tones. High key makes people feel happy, bright and pure, so it is often used by those who take pictures of children, medical work, and beautiful scenery; low key makes people feel solemn, serious and gloomy, so it is often used when taking portraits of the elderly or grand scenes.