Jacob already gave us an introduction to this text in the previous chapter (Jacob 1:17). In that introduction we were told that this was a sermon given in the temple (probably the temple precinct rather than the temple proper). Why does Jacob reintroduce the sermon here?
Jacob has created a new entry on the plates. In this section he is copying a sermon that was previously given. In this case, he gives us the text of the sermon and the very minimum of an introduction to create the context for this particular entry.