This is the only verse of this chapter that Moroni wrote, rather than copied. As he indicates, he is entering into his record a talk his father gave. Why might Moroni randomly add a sermon, and particularly, at this point in his record? Certainly, because he respected his father, but there is probably another reason. While Moroni has broken his book into chapters, there is a theme in chapters 2 through 7. Moroni has been laying down the way a church is to function.
In Moroni 6:9, the verse just previous to this chapter (if read consecutively), Moroni declared: “And their meetings were conducted by the church after the manner of the workings of the Spirit, and by the power of the Holy Ghost; for as the power of the Holy Ghost led them whether to preach, or to exhort, or to pray, or to supplicate, or to sing, even so it was done.”
Thus, we find that Moroni gives us a sermon, in a synagogue (or other gathering place) where the church would meet. He provides an example of the type of preaching and exhortation that he thought should be given in church meetings.