All Global System for Mobile Communication (GSM) handsets need a SIM card. It is a small chip that contains the data on your phone number, user ID, rate plan, billing information, and phone service. Its main function is to authorize you to make and receive phone calls. When you hear of a global roaming SIM card, it is simply a SIM card that can be used internationally.
When traveling, it is best to use international SIM cards rather than local SIM cards because it gives the benefit of having only one number. This is very advantageous especially if you are traveling to different countries in one trip. In addition, it offers cheaper rates compared to your local roaming charges.