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My global entry card conveniently fits in my wallet, unlike my passport which is otherwise not needed for US domestic travel.

Passport is presumably harder to replace if I lose--and hoses any upcoming international trips. You don't really need your Global Entry ID for anything so it's a good card to use for ID while keeping the cards you may really need later in the trip (or even just at the gate) somewhere safe.

The only time you need the Global Entry card is when crossing the land borders with Canada or Mexico where that card will let you use the special lanes while your passport doesn't. But for air travel, it isn't needed.

I always travel with two forms of ID, carried separately, in case one gets lost or damaged. Usually my NEXUS card and (real-id) drivers license. Passports are a bit more awkward to carry and can't go in a wallet (usa has passport cards but I'm not a us passport holder)

They can use it to lookup a boarding pass if you use the Known Traveler ID number from the card when booking.