Ramban: It is an obligation on the Levi'im to guard the Mishkan every night - to walk round it like sentries. In fact, the Kohanim guarded it from the inside in three locations ¹ and the Levi'im, on the outside in twenty-one locations. ²

Ramban:Refer to 3:6:2:1 & 3:6:2:2.

Oznayim la'Torah (citing the Ra'avad and the Rosh, at the beginning of Tamid and Midos): It is out of Kavod for the Shechinah who dwells in the Mishkan. ¹

R. Bachye: To teach us that the Yisre'elim were careful never to touch the Mishkan.

Alshich: Since they accepted to encamp in their proper places (in Pasuk 52), the Torah accredits them as if they had already fulfilled it. ¹