First of all, do you know what a SIM card is? As a traveler, you should.
A Subscriber Identity Module (SIM) card is a portable memory chip used mostly in cell phones that operate on the Global System for Mobile Communications network. Your SIM card holds your personal information, including phone number, address book, text messages, and other data.
If you want to change phones, remove the card from one handset and insert it into another.
Some travelers say to buy a SIM card once you land and switch over to it then.
Others say to buy one before you leave as you may need to use your phone before you have the opportunity to buy one. If you need to call another traveler to tell them your flight is delayed, if you need to call the hotel, or a variety of other reasons...having a SIM card already activated is a plus.
Getting one in advance may cost a little more, but you'll be ready to call once your plane lands.