For some, sketching may seem like an unnecessary step in the process of creating an effective User Interface solution. However, the utilization of sketching at the beginning of UI development can contain a lot of conceptual detail and opportunities for refinement. Sketching keeps the UI from entering the mockup stage prematurely and missing these important elements and modifications.
Sketching is where the basic concept is created, while refining that concept occurs during wireframe stage. UI Designers and even clients who rush too quickly to the graphical mockup stage risk proceeding in an inefficient manner where the concept doesn’t get the full consideration it needs to fully develop.