When you are in low light conditions, a flash can be essential for taking a picture. But, in some circumstances, a flash can also be helpful in broad daylight, especially for portraits.
When taking pictures while on auto-flash in the daylight, your flash may not trigger. Your smartphone or DSLR simply decides there is enough light and it doesn’t fire the flash. But even in the daytime, you can end up with a dark face if your subject is in front of a bright background, or have shadows on one side of a face if your main light source is coming from one side. Setting your smartphone or camera to fire the flash regardless of the ambient light can help light up a darkened face and/or eliminate shadows on your subject. This is call a “fill flash.”
While you only have the choice of on/off on your smartphone, but there will be tons of flash settings you can play with on a DSLR. Consider trying this trick next time you wanting a bit more light on your subject in a daytime picture. It can yield spectacular results, even on a smartphone.