The most distinguished among the Levites were the sons of Kohath, whose charge during the march through the desert was the Holy of Holies, and among the vessels particularly the Holy Ark. This latter was a dangerous trust, for out of the staves attached to it would issue sparks that consumed Israel's enemies, but now and then this fire wrought havoc among the bearers of the Ark. It therefore became a customary thing, when the camp was about to be moved, for Kohath's sons to hasten into the sanctuary and seek to pack up the different portions of it, each one planning cautiously to shift the carrying of the Ark upon another.
But this even more kindled God's anger against them, and He slew many of the Kohathites because they ministered to the Ark with an unwilling heart. To avert the danger that threatened them, God ordered Aaron and his sons to enter first into the sanctuary, and "to appoint to the Kohathites, every one, his service and his burden, that they might not go in to see when the holy things are covered, lest they die."
This was done because previous to this command the sons of Kohath had been accustomed to feast their eyes on the sight of the Ark, which brought them instantaneous death. But, according to this order, Aaron and his sons first took apart the different portions of the sanctuary, covered the Ark, and not till then called the sons of Kohath to bear the burden.