I first used them in Newfoundland on a woodland caribou hunt. Water everywhere, so rubber was important. But everywhere there wasn't water, there was gnarly low brush that destroyed footwear, especially rubber boots. There was literally a pile of ripped boots back in camp after the first day. I had gaiters and it did the trick. They took the brunt of the abuse.
When walking in brushy areas with rubber boots, it can be noisy as everything slaps against the rubber. Gaiters quiet things right down.
And as mentioned above, great for snow.