Completion

Hashem wanted to implant in the nation which He had separated for Himself a permanent mark to separate them from the other nations in the shape of their bodies, just as they are separate in the souls and their spiritual attributes, and by this the Jew completes his nature and form. Hashem wanted this completion to be done by man, rather than having the person born complete, to hint to him that just as his body is completed by him, so too must he complete his soul by his actions.

(Sefer HaChinuch)