This BBC article mentions that the signal showed that the two black holes rotated on distinct axes, suggesting that they did not form together (as their original stars, at least) in a single multiple star system, but rather formed a system later on. That such a thing is possible opens up a lot of possibilities for black hole mergers. It also provides another test of Einstein's theories, and the prediction that gravity waves are dispersionless. As the article says:
Also possible now are new investigations of the properties of black holes. The scientists can tell from the nature of the 4 January signal that the spins of the objects were not fully aligned when they came together.
This suggests they were not created from a pair of previously orbiting stars that exploded and then collapsed into black holes. Rather, their origin was more probably as stars that led independent lives and only at some end stage fell in as a duo.
"In that first case, we would expect that the spins would stay aligned," said Laura Cadonati, the collaboration's deputy spokesperson. "So, we have found a new tile to put in the puzzle of understanding formation mechanisms."
In addition, gravitational wave astronomy permits new tests of Einstein's theories. Because of the greater distance to this merger (twice the distance to the 2015 events), researchers could more easily look for an effect called "dispersion".
For light, this describes how electromagnetic radiation of different frequencies will travel at different speeds through a physical medium - to produce a rainbow in a glass prism, for example.
Einstein's general theory of relativity forbids any dispersion from happening in gravitational waves as they move out from their source through space towards Earth.
"Our measurements are really very sensitive to minute differences in the speeds of different frequencies but we did not discover any dispersion, once again failing to prove that Einstein was wrong," explained Bangalore Sathyaprakash, a LIGO team member from Penn State, US, and Cardiff University, UK.
The journal article itself can be found here.