On Stack Exchange sites, the field has no explanation for what it is used, other than it is "not shown publicly":
On Stack Overflow there is a hint text "Shown to employers only if opt-in":
Seems like this field is mainly used to reveal your real name to potential employers (or rather recruiters looking for potential hires on their behalf) via Stack Overflow job searching services.
What other purposes does this field have? What is it used for on the other 174 Stack Exchange sites?