Head circumference only tells part of the story
Without this sort of info, I’d essentially be guessing as to which shell would most closely approximate the shape of my skull – as would anyone else who might be in the market for a new helmet. This information would most obviously benefit someone shopping over the internet but even brick-and-mortar customers might at least like to be able to narrow down which helmet brands will work best before driving all over town.
I don’t have the data to say that a better-fitting helmet would protect you better than one that doesn’t fit as well (although it seems intuitive that the former case is preferable). However, I do know that I’m happier in a helmet that feels more like it inherently fits my head than in one that has to be adapted to work, and I’d rather not go through a big trial-and-error process to get there.