When it starts raining outside an incredible amount of water beats down on the roof of your house. Your roof does the job of protecting your house and everything in it from the rain. Most roofs are sloped so water can run off and prevent heavy collections of rainwater from gathering in pockets all over the roof. If this were to happen the roof could become severely damaged and eventually holes will form where the rainwater once was (it would now be underneath the hole that has formed).