Are there any games that benefit visually from turning fog off? The only games that come to mind that it would really matter on would be Resident Evil 4 and Meeroid Prime However, both of those games rely heavily on fog, so I'm not sure turning it off would really be a god thing.
imo, almost every game looks better with high res when fog is off. It often hides distant details that probably look terrible on a gamecube, but at 4x IR can be beautiful.
I found in most games, turning off the fog just makes everything look clearer.
Also in twilight princess, when using your senses in dog form, its a lot easier to navigate, since when using senses, it brings a thick, black fog to like five feet in front of you. Turning it off allows you to see a lot better than the developers had intended. Some monsters are easier to fight this way.
I'm going to guess in any shooter, it will allow you to snipe easier?
Disabling fog only disables a very small number of pixel shader instructions. It doesn't actually skip rendering of "far" objects, if that's what you expected. The only point in keeping that option is because, as RachelB pointed out, it increases visual quality in some cases. That's also the reason why it's in the "Enhancements" tab, rather than the "Hacks" one.