I first thought that the idea was to lend "fake credibility" to an image, by making it seem like the output from an application. But I guess the main idea must

I first thought that the idea was to lend "fake credibility" to an image, by making it seem like the output from an application. But I guess the main idea must be prototyping, and to show off how a particular design might look in the end, once implemented as part of an application.