Commentary for Numbers 4:20

ולא יבאו לראות כבלע את הקדש BUT THEY SHALL NOT COME IN TO SEE WHEN THE HOLY THINGS ARE WRAPPED IN, each into its wrapping, as I have explained above in this section (v. 5 ff.): ‘‘and they shall spread upon it such-and-such a garment”, “and they shall cover it over with such-and-such a cover”, — for the “wrapping-up’ of it mentioned here is identical with the “covering up” of it mentioned in those passages.

AND THEY [i.e., the Kohathites] SHALL NOT GO IN TO SEE THE HOLY THING ‘K’VALA’ (AS THEY ARE BEING COVERED) — “as they [the priests] put each vessel into its wrapping, as I have explained above in this section: ‘and they shall spread upon it such-and-such a garment, and they shall cover it with such-and-such a cover,’ the [term] ‘k’vala’ referring to its ‘covering.’” This is Rashi’s language. It is also Onkelos’ opinion [who translated the verse as quoted above]. But our Rabbis have said in Tractate Sanhedrin²¹⁷Sanhedrin 61 b. that this verse constitutes a prohibition against stealing a sacred vessel, for which one suffers death [by the hand of Heaven], and zealous people have the right to strike such a thief, for stealing and robbing are referred to as b’liah (swallowing, devouring), as in the expressions: ‘bala’ (he hath swallowed down) riches, and he shall vomit them up again;²¹⁸Job 20:15. and I will bring forth out of his mouth ‘eth bil’o’ (that which he hath swallowed up).²¹⁹Jeremiah 51:44.
But Rabbi Abraham ibn Ezra explained [the verse] in its plain sense: that [the Kohathites] should not come in to see when the screening Veil is removed and the ark becomes exposed; only afterwards when it is covered are they to come in to carry it. Accordingly, the expression k’vala eth hakodesh means [they shall not come in to see] “when the structure [i.e., the screening Veil] is being removed from the ark, which is hakodesh (the holiness),” [the word ‘k’vala’] being related to the expressions: ‘bila Hashem’ (the Eternal hath swallowed up) unsparingly²²⁰Lamentations 2:2. [which means that He “destroyed the structure” of the habitations of Jacob mentioned further on in the verse]; Thy hands have framed me … together round about; yet ‘va’tval’eini’ (Thou dost destroy me).²²¹Job 10:8.
Thus the Levites are warned not to touch the holy ark lest they die, and only by means of its staves are they to carry it. He furthermore warned them against coming in at all to see the dismantling of the structure, while Aaron is taking down the screening Veil, in a similar way to that which is said [of the people of Beth-shemesh that they were punished] because they had gazed upon the ark of the Eternal.²²²I Samuel 6:19. He [i.e., Ibn Ezra] has explained it well [i.e., the simple meaning of the verse]. But the [true] intention of the verse is that since the Glory that resides upon the cherubim [on the cover of the ark] is there, the Levites were warned not to break through “to see the Eternal”²²³See Exodus 19:24. until the priests take down the Veil, for then the Glory is seen in the hiding of His power,²²⁴Habakkuk 3:4. and it returns to its former place²²⁵See Hosea 5:15. in “the Holy of Holies.”²²⁶“In the Holy of Holies on Above” (Abusaula). In this way the expression k’vala eth hakodesh is to be understood in its literal sense. The student learned in the mysteries of the Cabala will understand.
Naso

ולא יבאו לראות, by having a previously assigned roster of individual duties you will avoid that someone will see things forbidden to be seen by him, as a result of which he would have to die. He will avoid watching the priests covering the holy vessels, something which would cause their death.