Plundering is used to loot resources from other cities. There are several reasons to plunder:
To gain resources faster than raiding dungeons. In the early stage of the game, dungeons are few and far apart. Sometimes it's faster and more profitable to plunder neighbors, and lawless cities. These cities are sometimes referred to as "farm." Dungeon adds a 2 hour round trip to a raid. There are also dungeon monsters that kill your troops and lower your profit. Your ideal targets are
- Defenseless cities of an inactive player or lawless cities, you can steal resources without any losses.
- Cities of players without an alliance. They may have some troops, but they won't get any support from anyone else. Their cities maybe weakly protected, and you don't countered from an upset alliance.
- Cities of players in a much smaller alliances than yours. If the cities have a lot of resources, you may still want to take the risk of a counter.
Be careful of plundering a big alliance. It can escalate into an all out war.
To suppress growth of a neighbor. If you have enemies around you, it's your best interest to slow their growth as much as possible. Plundering their resources can disrupt their building and recruitment queue and ultimately slow their growth. This can also be used as a tactic to prevent a continental invasion by another alliance.
To starve troops inside a sieged castle. When a castle is under siege, all the troops (including support troops from other cities) consume food from the sieged city, not where they came from. Some battles can involve millions of troops and consume 100k+ food/hr. The castle under seige also cannot make request for resources. So emptying out the food supply can starve out a lot of troops.
Example: Here's an example of a plunder report. This is one of my easy target that I plundered over and over again for resources.