Note that this is only the Zigbee Light Link master key. A lot of devices use the Zigbee Home Automation specification which has a different well known master key (in that case it's in the standard which is freely available).

The ZLL key is slightly more interesting because you can factory reset (and effectively steal) devices in someone else's network, but that does require physical proximity to the device.

The master key also means that you can make your own device to add to someone's network. Most ZLL networks have a simple push button adding process, so you just need to be close to the button for a few seconds in order to add your own device to the network, after which you can control any other devices already in the network.