To hold your gold Krugerands and raw uncut diamonds, of course. Just kidding. But they are primarily to hold money. There just isn't room for much else.

It's more important overseas, generally, where pickpockets are a bigger concern in many cities (especially if they can identify you as a tourist or foreigner). A money belt (I'm assuming teacher means just the belt, not the little pouches you can strap on under your clothes) won't hold all of your documents, but it does give you a pretty pickpocket-proof place to store some extra cash for emergency use. I've had friends/acquaintances have fanny packs (even worn in the front) and even one of those money pouches (it was under a sweater, but not tucked down into the pants) cut off or cut open by pickpockets.