De acordo com wikipedia , é para que eles não rolem fora:
A carpenter pencil (carpentry pencil, carpenter's pencil) is a pencil that has a body with a rectangular or elliptical cross-section to prevent it from rolling away. Carpenter pencils are easier to grip than standard pencils, because they have a larger surface area. The non-round core allows thick or thin lines to be drawn by rotating the pencil. Thin lines are required for high precision markings and are easy to erase, but thick markings are needed to mark on rough surfaces. The lead is strong to withstand the stress of marking on such surfaces. The pencil is robust to survive in a construction environment, for example when placed in a bag together with heavy tools. The core is often stronger than in other pencils. Carpenter pencils are also used by builders, because they are suitable for marking on rough surfaces, such as concrete or stone. This shape and lead density aid in marking legible lines with a straight edge that are clear and easy to follow with a saw blade.
Embora seja possível que haja algum precedente histórico envolvido:
The flat pencil is one of the oldest pencil types. The first versions were made by hollowing out sticks of juniper wood. A superior technique was discovered: two wooden halves were carved with a groove running down them, a plumbago stick placed in one of the grooves, and the two halves then glued together—essentially the same method in use to this day.
A teoria "plano para não rolar" também é suportada por este artigo em Lápis de carpinteiro em PencilPages.com
A carpenter's pencil has at least two essential characteristics which distinguish it from other types of specialty pencils. First, the shape of a carpenter's pencil prevents it from rolling, even when placed on a steep sloping roof. The most popular shape seems to be a flat, octagonal style. Secondly, the lead must be strong enough to make a heavy mark and to withstand the stresses of carpentry, which frequently requires marking on very coarse surfaces. The leads are usually wide and flat.