A “one sheet” (which typically is more than one page) helps you determine whats unique about your game and provides a good overview. You might create a dozen of these when pitching projects to a publisher and they typically don’t take too long to create.
When you are working on an internal/indie project this document comes out in the form of the game overview or something you might put on kickstarter. The purpose is to succinctly say why your game is different from others. What is your hook? The point here is not to get into the nitty-gritty but to capture the gestalt of the project. Here’s an example.