here is a program I just wrote that shows it’s usefulness!
void draw() {
if (mouseX < width/2 && mouseY < height/2) background(255, 0, 0);
else if (mouseX > width/2 && mouseY < height/2) background(0, 0, 255);
else if (mouseX < width/2 && mouseY > height/2) background(0, 255, 0);
else if (mouseX > width/2 && mouseY > height/2) background(255, 255, 0);
}