Apple has hidden an interesting twist inside the new Apple Watch: a built-in SIM card that isn't a SIM card. It's an eSIM, or embedded subscriber identity module. They've been talked about for years, but this is the first use of one in a mass market consumer device.
Essentially the eSIM is a SIM card that is permanently soldered to the device's board, removing the need for the plastic card that holds the chip, the locking mechanism that keeps it in place and the little tray that lets you swap it out. The resulting chip can be much smaller and perform the same job of providing the wireless hardware which network you're on, and so on.
It was originally intended for embedded devices in industrial and commercial settings. In this case, it lets Apple integrate wireless service without adding a SIM door or making room for a normal card, even the smallest of which is huge by the standards of the electronics in the Apple Watch.
But it's a new feature being adopted by Apple, so there must be a dark side. Like its other recent innovations, like removing the headphone jack, or removing all the ports from the MacBook but one, this takes control from the user's hands and puts it in Apple's.
The eSIM has to be provisioned via the device itself, which is handy if you want to switch between T-Mobile and AT&T in a hurry, but not handy if you want to switch to a carrier not supported by Apple, or sell your device to someone on a different carrier.
If you're okay with that trade-off, go for it. It's just part of the overall trend of Apple and others slowly removing standard features that offer user freedom.