Just to clarify a couple of points...
First, hoods block light at angles of incidence too shallow for that light to become part of the image formed by the lens. Without the hood, light at those shallow angles can enter the lens and reflect off lens element surfaces, decreasing contrast and saturation of the image. It might add slightly (probably insignificantly) to the total light in the image, but it would be added as diffuse, non-image-forming light.
Second, you should be using the correct hood for your lens. Canon's hoods are computer-designed to be matched to the lens. There are some caveats. Hoods for zoom lenses are effective mainly/only at the wide end (except the 24-70mm f/2.8L with its reverse-zoom design). Canon makes several EW-83 hoods with different single-letter suffixes, each design for a different lens. Although they will all mount on another lens that takes an EW-83 hood, only the correct hood will block maximal off-angle light without physical vignetting. One side note is that the hoods are designed for the image circle (although the petals of petal-shaped hoods take the rectangular frame into account). So, hoods for EF-S lenses are designed for the smaller image circle those lenses project, and hoods for EF lenses provide the best flare protection when used with a FF sensor, and are less optimal when an EF lens is used on a crop body. In those cases, you can improve flare protection by using a hood not designed for the lens, e.g. the EW-83J for the EF-S 17-55mm can be used on an EF 24-105mm f/4L IS when that lens is used on a crop body, and it will provide better flare protection than the appropriate EW-83H (but using the 17-55's hood with the 24-105 on a FF will cause vignetting).