What is the benefit of a Lens hood?

Blocks sun or other strong light sources to reduce/eliminate lens flare or glaring. They also provide a hint of protection for the lens since they stick out further than the lens itself.

Or more importantly, they make you look cool.

One came with my new lens. I'll give it a trial

Thanks

The block sun light and prevent flares. More importantly they also protect the glass from touches. I am so glad my 70-200 came with a lens hood. It is much better than my 10$ rubber hood for my 50 1.8.

It helps block (light) rain doesn't it.

They make your lens BIGGER

They prevent off-axis light from hitting the lens and flaring. Lens hoods are specifically designed that they don't get in the way, so using a lens hood will never be a bad thing, it's impossible for them to be in the way. They improve general image contrast by reducing off axis light, protect against flaring, protect against damage (much more than a filter protects), keep the front of the lens dry in rain (although you probably shouldn't use a non-sealed lens/body in rain anyway...) and stop stuff like dirt, dust or chipping or anything that might fly and hit the lens (obviously do nothing to stop on-axis chippings from hitting the lens). The only downside is if you accrue a collection of lens hoods, they can add some bulk and weight to a lens collection. They can however be mounted on the lens in reverse meaning they take up a minimum of space and I've never found them heavy. If they're included with the lens, in my opinion, they're something you really should use, and consider hoods for you other lenses.

1. Increases contrast.
2. Reduces flare.
3. Protects the lens.

I'm sold I will trial it out this weekend.