פּירוש אויף במדבר 3:6

ושרתו אתו [BRING THE TRIBE OF LEVI … BEFORE AARON] THAT THEY MINISTER UNTO HIM — And wherein consists their ministering?

ושרתו אותו. Seeing that the ultimate responsibility for anything connected with the Tabernacle was Aaron’s, as we know from Numbers 18,1 אתה ובניך ובית אביך אתך תשאו את עון המקדש ואתה ובניך אתך תשאו את עון כהנתכם, “you and your sons and the house of your father with you will have to carry the sin or guilt of the Temple; and you and your sons with you will have to be responsible for any sin/guilt involving the priesthood you have been appointed to.” As a result of these verses, the service performed by the Levites was in the nature of being assistants to the priests, comprising standing guard over the Tabernacle, protecting it against the entry of people not in a fit state for one reason or another. They had to ensure that the vessels of the Tabernacle remained not only in good condition but were not stolen, etc. The second half of that verse in chapter 18 refers to no non-priests being allowed within the sacred domain reserved for either the priests or even only the High Priest.

They shall preserve. These are not two topics — that they would serve, and also preserve their guardianship, because there is no other service aside from the guarding of the sanctuary which was the service of Aharon and his sons.

ושמרו את משמרתו THAT THEY SHALL KEEP HIS CHARGE — Because the care of the Sanctuary, viz., that no stranger should approach it, was committed to his charge, as it is said, (Numbers 18:1) "Thou and thy sons, and thy father’s house with thee, shall bear the iniquity of the sanctuary”, (which Rashi explains to have reference to the iniquity committed by a “stranger” in approaching the holy things), and these Levites aided them — this therefore, was their ministering [alluded to here].