First, most external flash units can move the flash head to "zoom" the flash, but not all can. Normally, matching the flash zoom with the focal length allows the light to spread as much as possible.
If you set the flash zoom to a longer setting than your lens it will darken the corners of the photo and enhance anything in the center. When using umbrellas, the zoom should cover the entire modifier without spilling over. When using softboxes the flash should be zoomed in to cover the entire surface of the modifier.
Concentrating the beam provides better illumination at a distance. When bouncing from walls or ceilings that act as a huge softbox, a concentrated beam may provide more reflected light, but a wider beam may be softer if the walls or ceilings are close.