You use your passport or green card to enter at airports. The only use for the GE card would be by cross by land back into the US if you want to use the Nexus or SENTRI lanes. It cannot be used heading into another country though. It's also good as a second ID or to confuse the TSA travel document checkers at airports. It's handy too because it doesn't contain your home address like a driver's license or evidence of travel history like a passport book.