I really dont see it as a big mystery at all. For instance, when hurricane Sandy hit, the only tree in our front yard, which was a fairly large tree, got literally blown down since the ground was so saturated with rain water. I heard the loud crashing noise from inside because I was literally only about 20 feet away from the tree right outside the window there. So its raining REALLY hard and the wind was about 50mph pushing the rain horizontal, and it was completely dark, so I needed to go out to check for any damage, which of course meant I NEEDED a waterproof flashlight.....
But aside from water, if you are using the light outside in summer in humid conditions that humidity can get into the light and cause corrosion over time. But also if you are out hiking and you cross a stream there's a possibility of dropping it into the stream.
Also, even if it isnt a huge possibility that you will drop your light in water or use it around water or rain, wouldnt you rather have a product that is built to handle such things? Maybe the better question would be, would you prefer a product that isnt well built over a product that is?