If David was busy with ban warfare, why did he take the plunder?
1 Samuel 27:9 (ESV)
9And David would strike the land and would leave neither man nor woman alive, but would take away the sheep, the oxen, the donkeys, the camels, and the garments, and come back to Achish.
According to Deuteronomy 20:16–18, cities that were under the ban were to be completely destroyed. Apart from Jericho (Joshua 6:1–27), it appears as if there was room for livestock and other goods to be taken (see Joshua 8:1–2).
9 And David would strike the land and would leave neither man nor woman alive, but would take away the sheep, the oxen, the donkeys, the camels, and the garments, and come back to Achish.