Fill flash

The advantage of using a fill flash located near the lens axis is "normalizing" exposure in situations like backlighting which exceed the ability of the camera to handle the contrast of the lighting.

Near-axis flash indoors and outdoors looks unnatural because natural light comes from overhead putting specular highlights higher on round surfaces like cheeks and creating downward and sideways shadow clues which, combined, help the brain interpret the 3D shape of objects in 2D photographs.

Bouncing a flash from a surface above the subject, such as a ceiling, creates the same downward modeling direction but is not a strategy that can be used outdoors.

The more ideal strategy for portraits is open shade or backlit which allows the subject to raise their face and eyes into the skylight without squinting.

The greater the amount of off-axis flash added, the darker the shadows will become after exposure is adjusted for normal-looking highlights.