Sketching is a low cost way to explore various possible solution paths. By comparing multiple quickly sketched possibilities, one can better choose the best path forward and and then iterate toward the best version of that solution with wireframing.
There are thousands of different directions that the design of an interface can take that would 'meet' requirements, but each such design has a varying maximum potential of acceptability, usability, accessibility, visual consistency, etc. One approach can be executed flawlessly, but if it is the wrong approach - it will still fall short. By sketching, wireframing and development time will be shortened when compared with a direct-to-development from requirements approach.
One of the most accepted methods for sketching is a technique called "Design Studio" which is a working meeting where stakeholders visually brainstorm to sketch out different interfaces ideas for a series of screens. Each person (concurrently and silently) sketches their ideas of what a particular interface could look like (timeboxed to 3-5 min). Individuals then pitch their sketch (1 min) and the group critiques each sketch (1 min), identifying pros and cons of each approach. After a round or two - an interface usually emerges that meets the requirements in the best possible manner. Most times it is an combination of various sketches and insights.
After exploring avenues by sketching, wireframing will be much faster. Dont worry about adding details to the sketches... they should have just enough detail to be understood.
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