Lens hood vs flower hood
While shopping for lens hood you will see two types of shapes, cylindrical and petal shaped. Cylindrical lenses are of different extended sizes, some are extended longer than the others.
Lens Hood with extended height
Lens Hood with normal height
The height of these hoods are designed to match the specific focal length of the lens as in wide angel lens if you are using the extended height lens hood it can interfere with the outer edges of the shots. It is mostly used with telephoto or longer zoom lenses.
Why using a Flower lens hood
Flower lens hood can serve the purpose of both lens. As you know lens hood of some specific size can be compatible with more than one lens of yours on which one lens might be a zoom lens and the other is wide angle lens. Using a flower or petal lens hood is that you can rotate it while using wide angel lens so that the edges (sides) are not effected with the extended height of the lens hood. While using on other lens you can rotate it to the desired place. You can say its two in one however for some people adjusting it for different shots might not be comfortable however I never tried to adjust it but still get satisfactory results with all kind of lenses.