Numbers 4:13

And they shall take away the ashes from the altar, and spread a purple cloth thereon:

The instruction was given for the time of journeying, when the tabernacle with all its sacred furnishings was to be taken down and carried forward. In that preparation the ashes were first to be taken away from the altar, and then a purple cloth was to be spread over it. This was one appointed step among the prescribed preparations for transporting the holy things.

Even so small a matter as clearing the altar and covering it was fixed by divine direction and was to be done in its proper order. The handling of the altar was therefore sacred work, to be performed with reverence and exactness rather than in a casual or indiscriminate manner. Nothing connected with the sanctuary service was left to human improvisation, but every movement was governed by order.

The work belonged to the divided service committed to the Levites, in which the care and carriage of the tabernacle was apportioned among designated workers. Each man was entrusted with his own particular task and was to perform it faithfully without seizing upon the task of another. The clearing and covering of the altar thus stood as part of a larger system in which sacred duties were assigned, not open to all alike.