The role of the prophet in war was to invoke the assistance of the Lord. In this case, Moroni needs to know where the Lamanite army is headed. They might be retreating to attack again at Jershon. They might go to Manti, surely that possibility had occurred to Moroni for the same reasons that it had occurred to Moroni. It was also possible, however, that they would take the more difficult route over the mountains to attack Zarahemla, or do something entirely different. For Moroni as well as any modern general, accurate information on the intentions of one’s enemy makes for an improved ability to combat them. Moroni turns to Alma who inquires of the Lord and learns that the next attack is to come at Manti. Moroni now knows how to deploy his forces.