There are many uses of Digital Twins in the real world, too many to list! Here are some of examples of digital twin technology in use today:

Spatial Analytics: this could be for safety, tracking customer behavior, traffic flow, visual effects (VFX), or manufacturing.
Example: Digital Twin | Stereolabs

Building Information Modeling (BIM): A complete twin of a building and all of its systems.
Examples: What Is BIM | Building Information Modeling | Autodesk

Predictive Maintenance: Digital twins can model and predict when a system is likely to fail and when to intervene before it fails.
Example: IoT based Predictive Maintenance Solution

Modeling and Visualization: Digital twins can be used to model systems large or small. These visualizations are useful for many purposes.
Examples:
Kroger Uses Simulation and Digital Twins for a Better and Faster Shopping Experience
Tyndall AFB’s Hololab debuts virtual gateway to Installation of the Future