A single pillbox would cause the column to fall out and go into battle formation, taking time and energy, and would likely be able to call in the column to headquarters to bring in artillery, airstrikes, or reinforcements. Even if it's taken out, single pillboxes are uncommon, so the enemy column would need to scout for other pillboxes nearby. Pillboxes deployed in groups would have overlapping fields of fire that would make it difficult to isolate and destroy any single pillbox, requiring more effort and inflicting more casualties on an enemy column.
