What is geo pinning?
Geo pinning takes a single ISO-2 country and applies it twice: the residential exit is chosen in that country, and unless overridden the browser’s locale and timezone are derived from it. Explicit locale and timezone fields exist for the cases where you genuinely want them apart.
Why it matters
The classic failure of "just set a proxy" is a German exit IP on a browser announcing en-US and America/New_York — a mismatch a site can trivially see, and one that no amount of proxy quality fixes. Driving both from one value makes the inconsistent state unrepresentable.