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Trapdoors can do the following:  Prevent falls. Prevent mob access to an area. Stop water, snowfall, rain or lava from flowing into an area. Act as an opening for a countertop, such as a bar. It still lets light come through and does not block redstone signals. This is the only way to place a trapdoor. Then you can build around it with other blocks. Right click on it. It will flip back onto the block it is attached to.
Use trapdoors in your structures. To place a trapdoor, put on the side of a solid block. Open a trapdoor.