Apart from the advantage of independence from a light meter, the sunny 16 rule can also aid in achieving correct exposure of difficult subjects.
The rule serves as a mnemonic for the camera settings obtained on a sunny day using the exposure value (EV) system.
The basic rule is, "On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight.
For example: Shutter speeds can be changed as long as the f-number is adjusted accordingly, e.g. 1/250 second at f/11 gives equivalent exposure to 1/125 second at f/16.
A change in the aperture of 1 stop always corresponds to a factor close to the square root of 2, thus the above rule.