There is another reason why a game concept is such an important milestone in starting to create a new game. Once your game genre, audience, and platform are defined, it is possible to take a look at what's out there in the market: your competition. Understanding games similar to the one you will be developing is crucial to the project's success. It is very likely that the developers of those other games have already faced and solved (or maybe not!) many of the problems you will encounter at some point during development.
Communities of players already playing those games might be discussing them in depth online. This is an invaluable resource for you, as you will be able to fix known problems or add features to your game, based on what real players are willing and hoping to play.
