I'd say 99% of the motivation is that it looks cool...
Sorry but that's 100% wrong.
After years of being blinded by the flat hood on my Montero I finally got sick of it and rattlecanned the hood black. It worked great.
Keep in mind that this is primarily for flat hoods. Hoods that are curved or that slope sharply away towards the front of the vehicle don't have as much of a problem.
But drive a flat-hooded vehicle towards the rising or setting sun and see how much fun it is. Your visor can block the sun, but it can't block the reflection coming up off of the hood. It's annoying and dangerous.
Also this is nothing new, I have a book called "The Off Road Handbook" that was published in 1975 or 76 that talks about doing this, that's where I got the idea from.
BTW I rattlecanned the hood black in 1999, long before I ever heard of ExPo.