Style guides and Wireframes (low to mid fidelity) serve as a foundation of the design process.  They give structure and consistency to the project and allow you to answer the follow questions to give your user a better experience.

1.  What do I want the User to focus on when they first view my site/app/interface?

2.  What information or functionality should be easily and immediately accessible to the end User?

3.  How should the information/functionality options flow as the User continues to scroll down and interact?

Like wireframes, prototypes are vary in degree of fidelity and are an essential part of user experience design process.

The experimental model of the website provides insights to less obvious areas of the user's world, and Improve time-to-market by minimizing the number of errors to correct before product release.