Matthew Poole and continuators on Numbers 4:5

5 And when the camp setteth forward, Aaron shall come, and his sons, and they shall take down the covering vail , and cover the ark of testimony with it:

For upon this necessary occasion the inferiour Priests are allowed to come into the Holy of Holies, which otherwise was peculiar to the High-priest, Heb. 9. 7.

The second vail, of which see Exod. 26. 31, &c. Levit. 4. 6. Heb. 9. 3. wherewith the Ark was covered while the Tabernacle stood, Exod. 40. 3.

Because the Levites, who were to carry the Ark, might neither see nor immediately touch it.