The title tells us who the speaker is ("The River Merchant's Wife") and what form the poem will take ("A Letter"). We're pretty clear about the first part, but this poem really isn't a letter, is it? It's more of a dramatic monologue, because you wouldn't normally write in this way to someone you know, no matter how flowery your style of language tends to be. The "letter" is full of descriptions that are highly stylistic and, well, poetic. It's a poem, not a historical document of communication between two Chinese people.
Another way of looking at the title is as a stage direction in the text of a play. Just imagine the description of a scene: "Enter stage right: a river-merchant's wife. Reading a letter." This fits into the monologue aspect of the poem. We don't get much else other than the action taking place.
Also, notice how, in the title, the speaker's identity is reduced to her relationship to her husband. She is his wife. We don't get her name, or really anything else about her. For that matter, though, the husband himself is reduced down to his profession: a river merchant. The title turns the characters of the poem into "flat," or two-dimensional, figures. In that way, the title lets us know that the focus on the poem really won't be on these two as characters. (And, true to form, the poem is really focused on the emotional experience of being separated from a loved one.)
By setting up the identity and profession of the husband and establishing that the wife is writing a letter to said husband, the poem is then free to focus on the details of loneliness. Thanks to this very straightforward title, the poem is able to dispense with the characters' backstories and go straight for the meat of their emotional experiences. That, plus sad monkeys. We mustn't forget them.