I assume (though am quite willing to be proven wrong) it is to be used as part of the die roll one has to make as part of Raid Resolution (latter part of point 2; though this is not explicitly stated to the best of my knowledge) on the Enemy Raid Flowchart, to see if the enemy gain a Morale, but I also use it as a means of randomly choosing a family member to use should game events call for it!
correct. part B of the enemy raid resolution can cause the enemy to gain morale if the die roll falls within the range of empty population markers. specifically, step 2 says:
Lose 1 Culture Point for each empty Population counter space on the affected Family's Box on the Population Display; –1 Enemy Ferocity. Roll a die: if die roll falls within the range of numbers below missing population; +1 Enemy Morale.
There's an example of how it works in the playbook at the bottom of the second column of page 12:
Step 2: We lose 1 Culture Point for each population counter missing from this family. We’re just missing 1 population counter from Fam- ily B—the one we just removed—so adjust Culture Points from 7 to 6. Next, we roll a die. If the die roll result falls within the range listed beneath a space which is missing a population counter, the enemy will gain +1 Morale. Because only a man is missing from Family B, a 1 or a 2 will result in an enemy Morale gain. For our purposes, let’s assume we roll a 4 for no additional effect. Finally, adjust the Enemy Ferocity counter from 4 to 3 on the General Records Track. That’s right! Enemy victories result in overconfidence.