Rhetorical: Benjamin is moving is logic forward very carefully. Beginning with the incontrovertible evidence of his work in service of his people, Benjamin is moving from the known past into the proposed new covenant. In that covenant, the people must accept a new or renewed relationship with God, so Benjamin is moving from the physical to the spiritual, from the tangible earthly king to the spiritual celestial king.
Verse 18 picks up on the theme of service in verse 17, and notes that this is an achievable goal. If Benjamin can serve them, surely they can serve one another. Benjamin is not asking anything of them that he has not already done, and to which they themselves were solemn witnesses (see verse 14).
Benjamin continues the logic by moving in a different direction through the servant/master relationship. He has begun by describing the relationship from master to servant – describing that which is required of the servant. Now he examines the relationship from the standpoint of the servant, viewed up to the master. Just as the people are grateful to Benjamin for the services he has provided (here not the work of his hands, but the removal of burdens “grievous to be born”). Benjamin moves that concrete example up to the level of the heavenly master. If Benjamin is the servant of the heavenly king, and the people give thanks to Benjamin, they should really be giving thanks to the heavenly King, for that is the master Benjamin serves. Rather than build a society based on the earthly king as master, Benjamin is moving the relationship to the heavenly king – with all benefits and responsibilities.