[Drugmachine]

Generally I am skeptical of acts that are done without reason, or the reason has been forgotten.
But like Mr. Venket said, some acts that we do during observing death, is not done at other times. I never found any reason, but I still follow it so as not to hurt the feelings of other family members.
Going back to your scenario, if I am coming in from rain and being wet into a dark room, I will light a lamp in wet cloths. It is the circumstances that dictates it.