A lens hood’s main job is to block stray light from outside the image area from entering the lens. That helps reduce flare and loss of contrast, and can also make dust or dirt on the front element less likely to be lit in a way that shows up in the image.

The petal (or tulip) shape is designed to match the rectangular image area. The corners of the frame “see” the lens at different angles than the top, bottom, and sides, so the hood can be longer in some areas and cut away in others. This gives more shading without blocking the actual picture. A plain round hood would either need to be shorter, making it less effective, or risk causing vignetting in the corners.

On longer focal length lenses, round hoods are more common because the field of view is narrower, so the special petal shape is less necessary.

As a bonus, a hood also offers some physical protection for the front of the lens against bumps, dust, and scratches.