Here's the second light rule again.
Sidelighting creates shadows—which cue our brains to see volume and texture.
The first intention of a painter is to make a flat surface display a body as if separated from this [picture] plane, and he who most surpasses others in this skill deserves most praise.
This accomplishment, with which the science of painting is crowned, arises from light and shade, or as we may say, chiaroscuro.
Leonardo da Vinci*
For example, if you're photographing the surface of the moon, do so when the moon is not full.
When the moon is full, the sun is lined up with the moon.
There are no shadows to show volume and texture.
Let's look at the second example.
You've probably seen wedding photographers who use their flashes off of the camera.
By holding the flash, the photographer can avoid the mug-shot lighting of an on-camera flash.
Shadows are created from the nose and chin, making the subjects more three-dimensional.
But, the flash doesn't always stay away from the camera for every subject.
When the photographer encounters someone with wrinkles or blemishes, the flash is held closer to the camera.
When the flash is near the camera, there are no shadows, and therefore, no texture.
Wrinkles and blemished are less noticeable without shadows.
For the third example, you have to imagine the scene without shadows.
The photograph of the Grand Canyon below was taken at sunset.
Therefore, there are shadows which help us to see the canyon as it really is—immense.
Imagine the same scene taken in the middle of the day.
Without shadows, the Grand Canyon would look flat.
Use sidelighting, to create shadows, if you want to show volume and texture.
Do the opposite, use light from the camera, if you want to deemphasize volume and texture.
Let's say you've hired a helicopter to take you over Monument Valley.
You'll be looking down at the buttes in Monument Valley.
When should you schedule the flight?
What are the two best times of the day to photograph Monument Valley?
Early morning or late afternoon light—because of sidelighting—are the best times.
The shadows will show the dimensions of the buttes.
* Kemp, M. (Ed.). (1989). Leonardo on painting. New Haven: Yale University Press.