An unlock code is simply a method to unlock your device that you purchased from one carrier so you can use it on another carrier. There's no added functionality as a result of it.
Posted from my CrackBerry at wapforums.crackberry.com
If someone needs an unlock code, it is not always necessary to purchase it. After being a customer of a carrier for some time, the code can be obtained from the carrier free of charge. How long it is necessary to be in the contract before getting a free unlock code depends on the carrier. You could find out from T-Mobile when you will be eligible for an unlock code if you need one.