Prototyping

Prototyping is an extremely useful tool in design.  In the field of architecture, people build prototypes or models to test for structural stability and aesthetic reasons.  Afterall, the last thing you want to have happen is spend all your time on an idea and find out too late in the process that it cannot accomplish the big ideas you had for it.  In video games the same is true.  Plus prototypes can inspire new ideas or perfect the current one.  The earlier you know how and what will work for your game, the better prepared you are to bring the whole thing together.