Lens Hood comes out to be very handy when there is too much of light in the environment you are doing your shoot.
The main use of lens hood is to prevent light striking the front lens reducing contrast and creating flare.
Flare basically occurs when light strikes directly on the object.
This light bounces straight on the lens.
Mostly this light is expected from very bright sources of light like studio lights or Sun.
When you use lens hood the pictures you take have good colors and a deeper saturation.
Another use of lens hood is to protect the lens.
Lens hood differ from size to size, not a single lens hood can fit to all lenses.
Mostly telescopic lens hood are in same size.
Lens hood are most often designed in such a way that they are easy to store.
They can be also fit to camera in a reverse position when they are not in use.