Albert Barnes Commentary Numbers 18:8

"And Jehovah spake unto Aaron, And I, behold, I have given thee the charge of my heave-offerings, even all the hallowed things of the children of Israel; unto thee have I given them by reason of the anointing, and to thy sons, as a portion for ever." — Numbers 18:8 (ASV)

By reason of the anointing - See Leviticus 7:35.