I have recently come across your problem, im majoring in English literature (university) what i have comprehended from this metaphor is that it leads to the character and person of the antagonist who is the major concern throughout this novel, namely Emma. She is into two basic habits of match making and literary engagements. It was also believed that Emma actually represented Jane Austen(the author) herself, as such Emma is the mouth piece of the author. As Emma is more of an intellectual elite she prefer to "letters"(relating to literature not the communicative device namely letter writing) more than the spoken word, and as writing a letter is a part of literary convention so the letters play major role in the novel. Another reason could be that in Jane Austen's age women did not socialize that much with men.So her knowledge about men in different situations, their psychological output was very scarce and spoken word requires the above said knowledge, so she prefered letters which did not require any such approach.