Sforno on Deuteronomy 4:41/ספורנו על דברים ד׳:מ״א

1אז יבדיל משה שלש ערים, after he had concluded with his introduction to the detailed discourse on the Torah. The reason why he set aside these cities at that time was to show the Israelites how the observance and carrying out of G’d’s commandments is of such supreme importance, and that he, though about to depart the earth, wanted to perform at least this commandment, the observance of which is linked to the land of Israel.