using a flash in sunlight

[crazyy]

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[Sherman Banks]

Well you could use a flash on a sunny day to eliminate harsh shadows. Also, if your flash carries enough power, you can use the sun as a rim light (illuminating from behind the model) while still exposing the face properly. There are many good examples of people using flash during mid day here if you browse the pool.

 

[KmH]

When a subject is backlit their face is in shadow. If you expose for the face the background becomes badly overexposed.

Using flash allows the photographer to balance the subjects exposure with the background so the background actually becomes slightly underexposed relative to the subject.

That better highlights the subject and provides for subject separation from the background.

 

[Dao]

You may search for "fill flash technique" in google.

 

[Big Mike]

As mentions, flash is often used in sunny conditions to fill in harsh shadows caused by bright sunlight and also to facilitate a balance between subject and background exposure.

There are some issues to consider though. When shooting outdoors in the sun, it's most likely going to be quite bright, so your 'ambient' exposure settings might be something like F16 & 1/200. That's all well and good, but the flash exposure is dependent on the aperture, and at F16, you will need a rather powerful flash to light up a subject more than a few feet away. You could open the aperture, to gain more flash range/power...but if you do, the shutter speed would have to go up to keep the same ambient exposure. The problem with that, is that you might be exceeding the max flash sync speed of your camera (it's probably 1/200 or 1/250).
The usual procedure for this, is to set your shutter speed at the max sync and then let the aperture fall where it needs to, for your exposure. Then you have to work within your flash's range.