Bright Sunny Days
You need a lens hood the most when the sun is at its brightest. Lens hoods are designed to block stray light from entering your lens. When direct light enters a lens from either the side or from the front, it causes lens flare, which will wash out your images. A lens hood prevents lens flare and results in better contrast in your pictures.
Indoors
Use a lens hood if you are shooting an event or in a situation where there are bright lights. Concerts, sporting events or any location where there is directional light can have the same effect on your pictures as sunlight.
Bad Weather
Attach a lens hood whenever you are shooting in rain or snow. In addition to using rain gear for your equipment, using a lens hood will prevent rain or snow from getting on the front of your lens and appearing in your pictures. Preventing rain or snow from hitting the front element of your lens will save you a lot of time in post-production editing.
Lens Protection
Put a lens hood on your lens for added protection to minimize the possibilities of scratching or even breaking the front element of your lens. Lens hoods, especially on longer telephoto lenses, protect the front element of the lens because the hood extends 2 to 3 inches in front of your lens.