Moroni had first explained the penalty associated with the promise (v. 10) because he was so vividly aware of those consequences. Here he explains the positive part of the promise associated with the land.
Literature: Moroni parallels both aspects of the promise: “For behold, this is a land which is choice above all other lands” (v. 10). “Behold, this is a choice land… ” (v. 11). Both clauses of the promise thus begin with the same preliminary characterization.