This is a good question. Concepts are important for any type of innovation to take place. To put it another way, the concept stage is the stage of creativity and theory. At this stage you do not worry about execution, you let your mind go wild and explore. For example, think of the concept stage before the personal computer was released or the Internet. It must have seemed crazy, but the one who thought about these concepts were geniuses. From this point of view, concepts are incredibly useful.
The prototype is the first execution of the concept. This is where the concept is made into a product or a model. During this time, it is not released into the public. There is continued testing to make sure that the product is working properly. During this stage imperfections are worked on and the product is refined. When the testers of the prototype are convinced that the prototype is ready to go, then they put it into production and use.