The power of the rain.
When it starts raining outside an incredible amount of water beats down on the roof of your house. Your house principally does the job of protecting your house and everything in it from this rain. Most roofs are sloped in order that the water can run off and prevent heavy collections of rainwater gathering in pockets all over the roof. If this were to happen the roof can become severely damaged and eventually holes will form where the rainwater once was (it would now be underneath the hole that has formed).
The damage that rainwater can cause.
If left to its own devices the rain water would run down the sloped roof and subsequently continue to pour down the wall of the house passing over window frames, windows, door frames, doors and possibly onto the damp proofing at the bottom of the house. Here it would collect in puddles that would take a long time to evaporate. All in all this would cause damage to the structure of your house, it can rot away wooden window and door frames and cause cracks in paving amongst other problems. During winter this damage does not take long to manifest itself and can cause further damage...