iTunes has the ability to update the carrier-specific settings on iPhone and iPad Wi-Fi +3G. Carrier settings updates are small files (about 10k) that are downloaded from iTunes to be installed on your iPhone or iPad Wi-Fi + 3G. The carrier settings can include updates to the default APN (how the device accesses the carrier’s cellular data network).
The carrier settings for iPhone also include special dialing codes at Settings > Phone > [Carrier] Services, default settings for Stocks, Maps, Weather, and other items.
It is recommended that you install these updates to your iPhone or iPad Wi-Fi + 3G as soon as they are available.

Related posts:

1. iOS 4: About over-the-air carrier settings updates
2. iPhone: About Cellular Data Network settings (viewing or editing the APN)
3. iTunes for Windows: iTunes cannot contact the iPhone, iPad, or iPod software update server
4. iTunes Store: About in app purchases
5. iTunes Store: About In App Purchases