What is a prototype?
A prototype is a model built to prove a concept.
This definition is applicable to anything, really, not only games. Most of the products we use every day, from the keyboard I am typing on to the complex machines that can fly outside Earth’s atmosphere and reach other planets, are the result of endless designs, prototypes, iterations, and, most of all, failures.
Game prototypes are not meant to be representative of what an entire game will look like; if that were the case, we would discuss instead tech demos or pre-alpha development builds. If you have a pre-alpha build ready, that means you are at a later stage in development and have probably already prototyped your core mechanics to get to the pre-alpha stage.
Nonetheless, even at the later stages, you might need to address problems that were not obvious before. Maybe finally seeing how your game actually plays in the hands of a playtester – or even your own – raises questions about...