When present, the LCD screen on the back of the Speedlite shows the zoom setting.
For example, if you take an image with a 50mm lens, the horizontal angle-of-view is about 40°. If the Speedlite were set to give coverage for a 28mm lens, the angle-of-coverage of the flash would be 65°. The Speedlite would, in effect, be emitting 25° of 'wasted' light, illuminating an area that is outside the field-of-view of your 50mm lens.
Conversely, if you set your Speedlite to its 105mm setting when you are using a 28mm lens, you will end up with a light beam that is too narrow, producing a spotlight effect with dark edges to the image.