When you don't have a reflector, and someone to hold it, fill flash is great.
But, fill flash may be too bright.
In the third photograph below, the brightness of the fill flash was reduced by one stop.
Exposure compensation is the feature with which you can adjust the exposure—lighter or darker.
Your camera probably also has flash exposure compensation.
Flash exposure compensation allows you to adjust the brightness of the shadows when using fill flash.
Flash was not used for the top pumpkin photograph, making the shadow too dark.
Fill flash brightened the shadow in the the middle photograph.
However, some photographers prefer a not-so-bright shadows.
Let's say you're doing a portrait.
If the fill flash is too bright, the person will look as if he or she were cut out and pasted on the scene.
That's because the quality of the light from the flash looks so different from the sunlight.
In the pumpkin example, I like the darker shadow in the third photograph at the top of the page.
The flash exposure compensation was set at -1.0.
Here are close-ups of the pumpkin at 0.0, -0.5, and -1.0.
Some photographers use their flash on a weak fill flash setting when doing portraits, whether there are shadows or not.
This produces catch lights in the eyes of their subjects.
Catch lights are the reflections of light sources on the surface of our eyes.
They add vitality to portraits.