However what other path for current is available here?
Water in a glass tank sitting on a stand that may or not be conductive sitting on a floor that is likely to be non-conductive.
In the example above, where the light fell in while a person was trying to open the top, consider the likely reaction of a person in that situation. It is likely that he would grab for the light to remove it quickly. Now, a person is in contact with the metal hood of the light and/or the water of the tank. Depending on what else that person is in contact with, you now have a fault path.
