Numbers 18:5 "And you shall keep the charge of the Sanctuary"

Guards stand outside a palace.

Soldiers patrol our borders.

So, too, the Beit HaMikdash had to be patrolled during the night (See Positive Mitzvah 22).

Beyond the protection this patrol afforded, it served as a sign of respect for the Beit HaMikdash.

This Negative Mitzvah forbids us to neglect the patrols which guard the Beit HaMikdash.