Zip-Code Lookup

The USPS has a form on their website where you can type in an address and find the zip code. Stuart Buck notices that the form has a place on it to enter the zip code. Why would anyone want to use such a form if they already know their zip code, right? Actually, I can think of one reason. It gives you the zip+4, which hardly anyone ever knows about their own address, never mind about other people's. It's still redundant, because there's no need for a zip code field if it can look up the zip code without the zip code, but it's not as if no one who could know the zip code to type it in would want to use such a form.