The iPhone, with all of its features through the iOS apps, is still primarily a cell phone.
This means that it still needs to utilize cellular service for making phone calls and sending text messages, not to mention that this is also what powers the 4G network for all internet connectivity outside of Wi-Fi. In the iOS 6, you have the ability to adjust very in depth iPhone cellular settings to accommodate your situation and preferences. Here is a look at how to adjust cellular settings on the iPhone.
To begin, go into Settings and head down to General, which is at the top of the fourth block of options. Now select Cellular from the top of the third block of options in General. You are going to notice that the Cellular settings are designated to three primary choices towards the top, with more specific settings at the bottom. The top one is simply the On and Off switch for Cellular Data. This determines whether or not you will have access to the 4G network for the Internet, or if you will only be allowed to use Wi-Fi. If it is turned On then it will use up data from your data plan if you have a limited plan, and if you are out of the country this can be especially bad for even unlimited plans. This is a reason that you may want to turn Off cellular data and make it so that you only have internet access with the Wi-Fi connection. When you turn it Off it will redirect all the functions that normally work on the cellular data automatically to Wi-Fi only.
The second option is the On and Off switch for Data Roaming, which is set to Off as a default. Data roaming is something that will occur when you are out of normal areas, like when traveling. This is when it cannot get a normal signal and has to “roam,” which charges you a lot more for the same data usage on the 4G network. This is not just true for international travel, but it can occur in different areas of domestic travel as well. If you keep this off you will be more likely to lose data service once in a while, but you will permanently avoid getting roaming charges. You could turn it on, however, if you know you will absolutely need to get Internet access at every single point.
The third primary options is going to be Set Up Personal Hotspot. What this does is set up your device to act as a mobile, wireless modem to a computer or other mobile device. Since you have a 4G data plan on your iPhone, with the Personal Hostpot you could plug in your iPhone to a computer with USB or Wi-Fi and then go ahead and use it as a connection wherever you receive 4G connectivity. This button does not enable a feature of you device, but essentially provides you a link to purchase it. The Personal Hotspot is something you can purchase from your service provider, but unless you want to add coverage to your account this is going to remain an irrelevant button.
Below this will be a final section under the heading Use Cellular Data for. These will be a series of On and Off switches for different functions that will then employ cellular data. This includes iCloud Documents, iTunes, FaceTime, Passbook Updates, and Reading List. iTunes and FaceTime will be set to the Off position by default. These are in this section because of their tendency towards high data use, and FaceTime wasn’t even allowed on cellular data until recently. It is good to look at your data plan before making a choice here. If you have an unlimited plan then there is really no reason not to have it turned to On for all five of them. If you, on the other hand, have a more average limited plan, then you may want to turn all of them off.