A passport book is always required to travel internationally. The only exception is that you may use a passport card in order to enter the United States by a land border crossing or at a sea port-of-entry. Passport cards cannot be used for international air travel. That said, passport cards are very useful forms of identification.
A passport card looks like a driver’s license and is considered an official government ID. You can use it as an alternative for a state-issued ID anywhere in the United States that requires identification. While you cannot use a passport card for international travel except as outlined above, it is a useful secondary form of identification. Many situations may require you to present multiple forms of identification.
Also, many travelers are understandably worried about having their passport book lost or stolen while traveling abroad. It is common for pickpockets and thieves to target international tourists for this very reason. A passport card allows you to keep an official identification on your person while you are out and about while hiding your passport book safely in your hotel or other accommodation.