iPhones are downgraded mostly to unlock. You see, with each new iOS, Apple also updates the baseband (like the phones modem) to higher version which is not unlockable with the available tools. So you have to downgrade to a lower version of baseband which is unlockable.<div>

<br></div><div>Luckily for iPhone 3G and older 3GS, you can always install the iPad 1 Baseband (0.6.15) which is unlockable with common tools. Although this affects GPS sometimes.</div><div><br></div>