Numbers 3:10
And thou shalt appoint Aaron and his sons, and they shall wait on their priest’s office: and the stranger that cometh nigh shall be put to death. And thou shalt appoint Aaron and his sons, and they shall wait on their priest’s office: and the stranger that cometh nigh shall be put to death.
Aaron and his sons were appointed to serve as priests and to wait upon the priest's office. That appointment replaced the earlier practice in which the firstborn son of each family held the priestly office. The Levites were assigned to other service of the tabernacle distinct from the priesthood. Any stranger or unauthorized person who came near the priestly work was to be put to death.
The priestly office and the sanctuary were holy and not open to anyone who chose to take part. The distinction between Levitical service and priesthood was divinely assigned, so to claim priestly authority outside Aaron's house was an act of rebellion. Elkanah, as a Levite, was therefore especially devoted to tabernacle service, though he had no regular employment there when its worship had become irregular and incomplete.