Mormon gives a more concentrated description of the nature of the Gadianton conspiracy here. The elements were present in the original introduction, but here they are condensed into a few verses where the information was spread over several verses the first time. For instance, Helaman 2:3 indicates that there was a covenant in the gang of Kishkumen that becomes the Gadianton robbers.
In Helaman 2:7 two members of the gang recognize each other through a sign. In other places we learn of the desire for robbing and plundering, and especially the subversive nature of the band. Since Mormon has already given us this information, his purpose in giving us these verses cannot be new information but rather a refocusing of information. Mormon brings the Gadiantons back to the forefront of his narrative, and must make certain that we know the essential nature of this subversive organization.