As Howell explained in an e-mail: "Let's say I want to make a map of a city by using people's heights as standard rulers. I just measure someone's apparent height, assume they are average height, and I can get a distance. If I knew the distribution of people's heights, I'd know how good this assumption was. Not every distance will be accurate (not everyone is a uniform height), but overall, I'll do pretty well."
He continues: "Since this is empirical, I don't need to know, for example, who each person's mother and father was to trust the result. But this information could help. If some of the people are Swedes and some are Indonesians, it is helpful to know that, since those two populations have different average heights."