Before a manufacturer may release a part or product to the public, it must go through a number of processes. While it may appear to be a linear process, many of these phases must be completed multiple times before the part or product can progress to the next. The prototype development stage is an example of such a step.
This is a crucial stage since it establishes if the product is fit for manufacture or function. It also saves money since the machinist can remove unneeded parts features at this stage. You should read this article all the way through if you want to understand what the prototype development stage comprises.