Enroute, even on a daytime VFR flight, you could encounter haze, smoke, marine layers, or flat light on snow-covered terrain, all of which can cause you to lose reference to the horizon. A fellow up here in Idaho was flying over a frozen, snow-covered lake on a gray day with no horizon and literally flew his plane into the snow. He had no idea how low he was, similar to a glassy-water situation for a seaplane. Fortunately, he was fine, although his plane was totaled.