When you're using a Canon Speedlite, it's usually recommended to match the flash output to the focal length of your lens to make the best use of the light. Helpfully, some Speedlites have an auto zoom feature that does the job for you, ensuring that the flash coverage corresponds with the angle-of-view of the lens. This means the light spreads across the whole image with no 'wasted' light where the flash illuminates more of the scene than is captured in the image.
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.