Let’s build an IoT application with weather sensors deployed around the globe. The sensors will collect data and we store the data along with the IDs of the sensors. We’ll run multiple database instances, and the sensors will write to the geographically closest database. All databases will regularly exchange data, so all the databases will eventually have data from all the sensors.
We need each sensor to have a globally unique ID. How can we achieve it? For example, we could run a service assigning sensor IDs as a part of the sensor installation procedure. It would mean additional architectural complexity, but it's doable. Sensor IDs are immutable, so each sensor needs to talk to the ID service only once - right after the installation. That’s not too bad.