Deuteronomy 18:6

Levite. In the days of Moses, all the Levites probably assisted in the service of the tabernacle, when they thought proper. But, after they should be dispersed, he encourages them to come willingly. David afterwards divided the priests and the Levites into classes, which were obliged to serve in their turns, 1 Paralipomenon xxiii., &c. Though he derogated from the words of the law, he followed the spirit of the injunction, which was intended to promote the great glory and decency of religion; and even after this regulation, (Calmet) those who desired, like Samuel, (Menochius) to consecrate their labours to the Lord for life, or for a long time, were in all probability entitled to the privileges here granted. (Calmet)