Remove a SIM Card? Why?
Ever since the dawn of the modern cell phone, providers such as AT&T, T-Mobile, and Verizon have used little devices called "SIM cards" in order to prevent people from using their cellular phones who weren't enrolled in one of their plans. For example, traditionally, a SIM card would prevent someone who was a Verizon Wireless customer from using an AT&T, T-Mobile, or other company's phone. In addition, a SIM card can store useful information on it, such as your phone's contacts, texts, and other things that can tell your phone about your custom set-up.
So why would you ever want to remove one of these? Well, the thing about SIM cards is that inserting yours into a phone (that is compatible with it, of course) allows you to actually use the phone under your name and with your contacts. No SIM card means no phone usage, so if you buy a new phone and you want to use it, you have to take the SIM card out of the old one and put it into the new one. If you switch to an iPhone, you need to know how to put in the SIM card, and if you switch from an iPhone you need to know how to take it out. It's as simple as that!