Why do professionals use charcoal instead of pencils for sketching?

Regular school type pencils don't have the necessary graphite content to get very good contrast. The typical school pencil also doesn't have the longevity needed for art.

Artists use pencils to do their sketches and line variety and to work out ideas that can be easily erased. Regular #2 school pencils are great ways to begin your art! And if you get enough pencils together in one place, you might have enough wood and graphite to build yourself a nifty cool pencil sculpture! Pencils also create Lines.... the most basic element of art.

Pencils do not need to have graphite, pencils are nothing more than a tool to make a mark onto something. Ever go to the beach and draw in the sand with stick? Your stick was your pencil!