It could be a long while before we see any practical uses of the breakthrough. It's expensive to create the necessary precursor molecules, and it's not yet possible to get cells making those precursors all on their own. Should the processes become cheaper and easier, though, extending DNA could be helpful for testing the effects of medicine on certain gene combinations. It could also be used to deliver gene therapy; you could have tailor-made DNA that combats specific problems. Whatever comes next, it's now obvious that nature has a pretty limited vocabulary.