In real life, if you turn on a bright light in a dark room the entire room will instantly* light up. In games that use RTGI solutions with slow accumulation, only part of the room will instantly light up and the rest will only light up after a noticeable delay. If you turn off the light, it will instantly turn dark in real life, but there will be a delay with RTGI and accumulation. This is actually noticeable in Metro Exodus EE.
*Technically there will be a delay based on the speed of light, but that might as well be instant as far as human perception is concerned