The Sun in Your Pocket
Depending on available light when photographing people directly in the sun often causes subjects either to blink or squint their eyes. Using a flash, which is an instantaneous light source, eliminates both blinking and squinting faces.
The internal flash on your camera is a point light source. If used alone as the main light source, it creates shadows that make the portrait look unnatural.
Most of us are used to thinking of using the camera's flash only at night or in low light conditions. However, it's very useful in the daylight too for subjects under the shade of a tree or photographed with the sun at their backs. The blink of the flash in such conditions creates a lively shine in the eyes and freshens the colors of the skin.
For more details check out The Sun in Your Pocket catalog available in PDF format.