I think that two men who are good friends ought to front name one another.
Benjamin Tanner says this to Robert Peck as he arrives at Haven Peck's funeral. As the Tanners' wagon pulls up, Robert greets his neighbor as, "Mr. Tanner," which is what he had been calling him throughout the book. When Mr. Tanner asks Robert to call him Ben, it is a confirmation of Robert's manhood. Whereas he had been a young man before, the way that Robert handles his father's funeral is the final step in the transition. Mr. Tanner makes Robert his equal by asking him to call him by his first name, which gives Robert confidence.
This quote also demonstrates that Robert has accepted his role as the head of the family and as a Quaker farmer. Earlier, when Mr. Tanner explains to Robert that farming is the highest calling to which a man can aspire, Robert is not convinced and still thinks of using his education to do something else. Mr. Tanner would not let a thirteen year old stand on equal footing with himself if Robert had not come to understand this.