When the chiaroscuro technique is used in painting?

Chiaroscuro is Italian for light and shadow

Painters use this technique when they need to create the illusion of light on objects and  people

I've attached a pic for example :D

The linear perspective changed the quality of art because it gave art a sense of reality. it gave the illusion that the background and surroundings were actually close and/or far and a sense of depth. linear perspective change the quality of art following its birth the renaissance period as they want the art to be more realistic and they believe one way to achieve it is to use the method of linear perspective. it uses the principle of math to realistically portray space and depth in art.