The reason for using a hood is to block light rays from elements
outside of the actual frame (stray light) to enter into the lens
and degrade the image quality (lowering contrast). Effectively,
it "shades" the lens from these rays. Since a sensor is
rectangular, valid light rays that fall on the corners arrive
from angles larger than light rays that fall on the middle of
the sensor's edge. This is why many hoods have the "petal" form.
Otherwise you will suffer from either vignetting at the frame
corners, or lower effectiveness in blocking invalid rays. With
increasing focal length, the petal shape becomes less required
and there is a transition to circular shape. Another reason for
circular hoods is rotating front elements of the lens, where the
hood is mounted. When focusing, the element (and hood) rotates
so if you use petal hood it will lose its alignment with the
sensor.