According to the paper, they did it in order to figure out why SARS-CoV was able to infect humans, while numerous other SARS-like viruses are not. Their hyposthesis was that the critical feature is a particular sequence in the protein found on the "spike" that the virus uses to bind to host receptors. By constructing a new SARS-like virus with the desired spike protein, they were able to validate that hypothesis.
It seems to me that if there was any malign intent behind this work, they would not have published a paper on it. Obviously, that's doesn't preclude the possibility of an accidental release from the same lab twelve years later. As far as I know, there's no way to prove whether 2019's coronavirus was engineered in any way.
I want to repeat what I've said in other comments, because what I initially said was inaccurate. These researchers didn't actually create a new coronavirus, but rather a much simpler and safer virus purpose-built to test some receptor-binding structures. This is a well-established way to study viruses safely.
This kind of work is critical if we're to learn how to develop vaccines faster in response to epidemics.
It seems to me that if there was any malign intent behind this work, they would not have published a paper on it. Obviously, that's doesn't preclude the possibility of an accidental release from the same lab twelve years later. As far as I know, there's no way to prove whether 2019's coronavirus was engineered in any way.