It’s only a pencil and Apple has omitted the obvious
Should you happen to have a couple pencils in front of you, one with, and one without rounded edges, you can see for yourself. With intensive drawing, your fingers begin to sweat, and the standard №2 pencil provides a better grip.
The heptagon or seven-sided polygon determines the shape of standard pencils. It enhances the grip when drawing or writing, especially when intensified and the finger begin to sweat.
The enhanced grip is unavailable with the Apple Pencil for iPad Pro, a reason why you’ll find pencils with round shapes less often than ones with rounded edges.