The formula example may not be the best use of the functionality – but it is a way to use it. The example also indicated that entering =customers will display the name of the customer in the range, in that row. This would indicate that using the formula in a row outside that range would produce an invalid result like you were seeing.
I suppose another formula example could include a reference to sales figures rather than customer names, where a Named Range can be added up using =SUM(SalesByCustomer) or some other similar range. Perhaps even using it to track inventory would be another good example =SUM(BeginningInv,PurchasedInv,-SoldInv).