Sforno on Numbers 8:14/ספורנו על במדבר ח׳:י״ד

1והבדלת את הלוים, this “separation” will express itself in their having separate areas in which they would encamp, i.e. the Levites who were members of the people who had participated in the Exodus and their children growing up in the desert;

2והיו לי הלוים, and both they and their descendants, (when they would not all live next to the Temple) would be for “Me,” i.e. ready and prepared to carry out their duties as Levites.