If you live where it snows, imagine stepping outside after the first snowfall of winter.
The snow makes your neighborhood look different.
You'll notice different things than you would have the day before.
The snow on the rock below changes how it looks.
The snow, the "light," makes the surface of rock more apparent.
Sunlight, coming from the side, would do the same, by creating shadows.
But, we wouldn't notice the affect of that particular type of lighting, as much as we would the affect of a dusting of snow.