Wheatley's choice of heroic couplets in "On Being Brought from Africa to America" helps to underscore the poem's key message—that black people (with Wheatley herself as an example) are not hopeless or doomed, but can be saved and even "refined" by Christianity.
Heroic couplets have been a feature of English verse since at least the fifteenth century, when Geoffrey Chaucer used them in the Canterbury Tales. They became intensely popular in the late seventeenth and early eighteenth centuries, when they were featured in the works of Alexander Pope and John Dryden.
Wheatley was born approximately ten years after Pope died and about fifty years after Dryden did, which means that she was born into an era in which Pope's and Dryden's work had reached a wide audience (for the time) and was considered a paragon of fine literature. By using heroic couplets, Wheatley places her own poem in this tradition of fine literature. It's another way of saying, "Look, black people like me can create what white people consider fine art, too."
Wheatley's audience is comprised of Christians, particularly white Christians. She addresses "Christians" directly in the second to last two lines: "Remember, Christians, Negros, black as Cain / May be refin'd, and join th' angelic train." Since she positions Christianity as the means by which black people may be "refined," we can infer that Wheatley is not addressing black Christians specifically, since according to her own argument, a black Christian would already be aware that they could be "refined" by embracing Christianity.
One could argue that there are two different ways to read this poem. The first is as if Wheatley is speaking earnestly to white Christians, encouraging them to see black people as both capable and worthy of redemption through Christ. The other is as if Wheatley is slyly poking fun at white Christians' beliefs about salvation, saying to other black people, "Look, they think if we use their version of English and their poetic forms, that's the same thing as being human (but we know we were human all along)."
I, personally, have never found this poem shocking so much as unsettling. It's very easy to read at face value, but its face value borders on pandering. I personally prefer the more coded reading, simply because I don't want to believe that Wheatley really did see herself as "benighted" or "diabolic"—though of course I have no way of knowing how Wheatley really did see herself.