• When the subject is in shadow, such as these blooming dogwoods:

     Flash: Dogwood blooms, Yosemite National Park, California

• When you want to highlight the foreground or subject 
• When the background is brighter than the fore ground
• To reduce shadows being cast by facial features 
• Or, to add catch light to the eyes of the subject, like this very handsome giraffe:

    Flash: Giraffe, Tanzania

• It balances out the scene 
• It gives attention to the subject by illuminating it properly, like the design in this coffee beverage:

    Flash: Coffee beverage, Sartory, El Dorado Hills, California

• And, it adds dimension, such as in this shot of a risotto dish:

    Flash: Risotto, El Dorado Hills, California

• A certain amount of experimentation is required and you will begin to notice when and how much is needed with time and practice. 
• I typically expose for my background, then do a test shot with the flash at 100% power. Note, most cameras have a maximum shutter speed of 200 when using fill flash. 
• If the fill flash is too strong, it will have an unatural and overexposed look to it. To adjust, simply dial down the power and try again until you achieve the results you desire. 
• You may need to adjust exposure settings to ensure your foreground and background are balanced out as well. 
• I have an off camera shoe cord that I use to move the flash around so I can do side, top, or low lighting. Some people use transmitters to do this wirelessly as well.