The wisdom of Gidgiddoni’s plan is discovered in verses 2 and 3. The Gadiantons were an army. They had marched into a new land. A problem of armies is supply, and most important in the supply line is food. While they can carry enough food for a time, an extended campaign, such as a campaign of conquest, requires that the food supply be replenished. While that might happen from the homeland, it was typical for armies to confiscate food in the lands they were conquering. Except, there was no food to scavenge. Although the text only mentions animals, it is certain that Gidgiddoni also had his people take all of the grains they had with them, and if they couldn’t carry it, or it wasn’t sufficiently ripe, to destroy the fields. This ensured that the Gadiantons couldn’t scavenge the land.
This leads directly to verse 4. Without food, they are required to return to their mountain strongholds. The next question would be whether or not the Gadiantons could simply take over the fields. They certainly could have taken over some of the fields, but the Nephites could destroy those fields with small sorties. The result was that there was a large buffer zone between the Nephites and the Gadiantons and the Gadiantons couldn’t safely occupy the now deserted territories to begin the process of growing again (see verse 6 below). That would take time that the Nephite sorties would not give them.
Even though the Nephites had the stores to survive a seven-year siege (verse 4), it was in the next year that the Gadiantons chose to attack.