An eSIM is a SIM card built into the phone and provisioned by software rather than a card you insert. Manufacturers are removing the tray entirely in more markets each year.
Why manufacturers want it
The tray takes space and is a hole in a water-resistant body. Apple noted that eSIM-only iPhone 18 Pro models get extra battery capacity in the space the tray occupied – roughly two additional hours of video playback.
That is a real user benefit and it is also convenient for manufacturers.
The genuine advantages
Switching carrier without waiting for a card. Adding a local plan while travelling, in minutes, from the airport.
Multiple numbers on one phone without a dual-SIM tray.
And it cannot be removed by someone who steals the phone, which makes tracking more likely to work.
The significant downside
Moving your number to a different phone is no longer taking a card out. It requires the carrier, an app, sometimes a QR code, and occasionally a phone call.
If your phone is broken, lost or has no power, that process can be considerably harder than swapping a card into a spare handset.
Travellers who relied on buying a local SIM at a kiosk find the eSIM equivalent less universally available, particularly outside major markets.
What to do
Before travelling, check that your carrier supports eSIM transfer and know how to do it. Save any QR codes you are issued.
And if you keep an old phone as a backup, confirm you can actually move your number to it in an emergency, before there is one.