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 a project, especially a project that includes new concepts or ideas that might be hard to get fully coded and work as intended. You might have heard of the term proof of concept; this chapter is exactly what that is. It's a very quick overview of...