Flash close to the subject allows you to use small lens openings to get the depth of field you need for close-ups. When using flash, it's a good idea to use the fastest sync speed available on your camera. Using a combination of a fast shutter speed and small lens opening with flash lets the background go very dark because there is so little exposure from the daylight. You can use this technique to tone down a distracting background or to make a bright, colorful subject stand out against a dark background. Shooting at fast shutter speeds also minimizes subject movement, such as the swaying of flowers on a windy day.
Electronic flash is excellent for close-ups of live subjects to stop motion.
Outdoors though, your subject must be in dim lighting, such as in deep woods, in order to use electronic flash to stop the action. Dim light is necessary so the flash will overpower the natural lighting and become the main light source. Determine the exposure the same way as you would indoors. When you make extreme close-ups with flash, it simplifies exposure calculation if you can use your flash off the camera, always at the same distance from the subject. Then regardless of your subject distance, your basic flash exposure with a specific film and flash combination will always be the same.