Numbers 6:16

And the priest shall bring them before the LORD, and shall offer his sin offering, and his burnt offering:

The priest brings those under the Nazarite vow before the Lord and offers the required sin offering and burnt offering as part of their purification and of completing the vow. Finishing the vow depends on this priestly presenting of the person with its prescribed offerings. At Jerusalem four men still lacked these purification sacrifices, and until they were offered their vows remained incomplete.