I cannot think of any reason why any map would be useless. All maps, if they are accurate, are important pieces of information that help to create a complete picture of the world. Even if there is a place where no one lives, no one ever travels, and no one even talks about, it is still important to map the area. If we had holes in our maps, we would not know where build in the future. We also would not know where we should not build.
For example, even a swamp is important to map if it contains protected wildlife and wetlands.