Besides the obvious reason of asking your friends "is it fun?!" before it's fully complete, prototyping your game, especially quickly and early on in development, can be very useful for a few different reasons:
- You can ask about the originality/innovation of your game from the perspective of an end user instead of just your own views
- You can generate ideas on how to improve the game way before it's too late to make changes
- You can get a feel of how the game actually flows from one stage to the next, and conceive a tangible product instead of just an idea
- If shown to the public, it could be a great way to begin the marketing of your game and beginning the snowball of exposure needed to succeed on iOS
Plus, this is the best way to start ...