Charles Ellicott Commentary Deuteronomy 18:6-8

"And if a Levite come from any of thy gates out of all Israel, where he sojourneth, and come with all the desire of his soul unto the place which Jehovah shall choose; then he shall minister in the name of Jehovah his God, as all his brethren the Levites do, who stand there before Jehovah. They shall have like portions to eat, besides that which cometh of the sale of his patrimony." — Deuteronomy 18:6-8 (ASV)

And if a Levite come. — The Levites with the priests were to receive forty-eight cities in Israel, with their suburbs (Numbers 35:7). There was still no provision made by which all could serve in turn at the tabernacle. When David divided them all into courses—priests, Levites, singers (and porters?) alike—there was no longer any need for this provision. The institutions of David prove its antiquity.

The only case in history that illustrates this is that of the child Samuel. His father, Elkanah, was a descendant of Korah. He lived in Mount Ephraim and came up to Shiloh year by year. But Samuel was dedicated by his mother to perpetual service there. As long as the tabernacle continued in Shiloh, the child Samuel ministered to the Lord before Eli the priest—not as a priest, but as a Levite in attendance upon the priests.