Petal-shaped hoods are designed to block as much stray light as possible without appearing in the frame at the lens’s widest angle of view.

Because the image area is rectangular, the lens can tolerate a deeper hood at the top and bottom (and sometimes sides) than it can at the corners. A round hood deep enough to give the same shading would vignette the image corners on the wide end. The petal cutouts remove material where the corners need clearance, while leaving more hood depth where it can still help.

This matters most on zooms: the hood is usually designed for the shortest focal length, since that is the most demanding in terms of avoiding vignetting. That means the hood is a compromise and is less effective at the longer focal lengths than a dedicated hood for just the tele end would be.

So the cuts are not there to help by themselves—they let the hood stay deeper overall without intruding into the picture.