Flash Value Lock (FV lock)

Subject not in the center

If using flash, it will weight to the centre and the subject will have the incorrect exposure. What you do then is focus on the subject and use the FV lock, let it do the preflashes and lock the flash power. To unlock it, push the FV lock again or set the power off. If you wouldn't use FV lock the flash would lock on the center background and if that is for example a dark landscape would use full power and overexpose your subject horribly.

Shooting several people

Using FV lock will use the same power on all of them.

Fast blinkers

People that blink on the preflash and their eyes are closed on the main flash.