As an artist who started sketching and ended up using only a computer, it could already be a question if using a pencil to lay out is even necessary. Apparently, you could already develop a workable design with the use of tools available in whatever software you use for visual art. Moreover, for others, sketches would only take much of their time if they could work right away in front of a monitor.
However, there are reasons why you should walk the traditional path. First, sketches spark creativity. Although this is applicable to a graphic designing software, a pencil gives more freedom to illustrate what is in your mind. By simply beginning with lines, points, and shapes, you are walking towards an idea that will soon pop out of your paper.
It is naturally difficult to explain a visual artwork to a client if the concept is complex, and sketches could make it simpler. Without the need to finish the work right away, a designer could give his or her client an insight of what he or she would do. Moreover, the other party would be able to focus on the core construct of the design instead of colors, fonts, and overall arrangement of elements.
An art director is also enabled to provide directions instantly to a copywriter or graphic designer by sketching. He or she no longer need to present his or her gadget simply to illustrate what has to be done.
Sketches eliminate the touch of computer variations, which at times, leads to an altered abstract or output far from your initial plan.
Most importantly, sketches keep your image as a graphic artist intact. Truth to be told, you would discover real talent on handwork than on virtually published artwork. There are people who do not necessarily have to master how to navigate a software simply to finish a design.