Most likely, those notches are mainly cosmetic rather than functional. On a fixed beveled edge, knurling can be used to give the lens a more refined look and to echo styling found on older lens designs, where similar front-edge detailing was more common. Manufacturers sometimes keep such visual cues as part of a brand’s design language.

There may also be a minor durability benefit: a knurled or notched surface can make wear less obvious over time. Instead of a smooth painted bevel developing a continuous shiny worn line along the edge, small raised and recessed areas tend to break up visible scuffing.

So the practical answer is: mostly aesthetics and design continuity, with a possible secondary benefit of helping hide abrasion.