An extremely interesting and highly debatable topic you picked.
Just last Friday while I was riding with an IMC recovery guy, who is an ofroader too, he bought a snorkel for his Pajero and asking me the same question - how it helps and why so many people have them here in UAE?
Although I'm yet to see Amazons jungle and water wading opportunities here in UAE except a couple of weeks in some remote wadi's during the rainy season. In such water wading that has more than 2 feet deep water, such snorkels are helpful so that air intake which is usually placed under the front fender doesn't suck water and stall the engine.
Apart from water wading snorkels practically has no use and instead bring a little toll for sand offroading and cause a restriction and a delay while sucking air through a long tube. Imagine you are breathing with a straw while running. However, this can be managed to some extent, by reprogramming the ECU to adjust the intake long travel.
I agree they look cool and some people I know personally they have fake snorkel attached, just for looks without any alteration of intake and air filter modification. Just like the two antenna mod started with Shkmo on his G500 decade ago, now followed by every G and non-G cars and accessories shops even sells a fake stick on antenna these days.