Well, there's some that registered before those fields were put in place, and a few that have to setup manually for various reasons so it's possible some get missed that way.
The main reason the fields are there is twofold:
1. To help folks find other members in their general area that might be able to help answer specific questions. Or maybe find someone thats in your area that would be willing to show you their kitchen

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2. So we can eventually tailor newsletters by region to let folks know when something special is happening in their area.
I know some folks feel uncomfortable filling in all the fields, but they really are there so we can do more helpful things in the future, we never give user information to anyone else, and we take member privacy *very* seriously.
James