The siege at Cumeni did not consist of simply waiting to starve out the inhabitants. According to Hassig, “The city itself might be safe, but its fields and stores beyond the walls were still vulnerable. So were its smaller unfortified dependencies, and without these the city was lost anyway. In consequence, a static defense was a losing one. Only an active defense that defeated the enemy would enable the city to continue as the hub of a social network.”
The Lamanites made sorties at night, attempting to breach the siege-circle, when the majority of the Nephite army was asleep. If these sorties could identify and exploit a weakness, they might open a path through the Nephite army and break its stranglehold. The Nephites were prepared for such attacks and repelled them.