The starting point for any creative endeavor is an idea. That idea is then built into an outline, fleshed out and turned into something so much more. In the case of video games, that old notebook often takes the form of a prototype. These prototypes are often kept under lock and key, hidden by creatives who dread to think what Johnathon Public would think of their broken toys. But sometimes these ideas need a fresh set of eyes to turn them into something more concrete. Or maybe an excited youngster, not yet ground down by the harsh realities of the industry wants to show everyone their great idea. Perhaps the idea didnt pan out for a developer, and they want to share it with the world before walking away as proof of life, some kind of beacon indicating theyve done something.
Source: N4G PC Exploring 3D Platformer Prototypes The Good, The Bad, and The Forgettable