With the desktop app and the web client, it has been possible to use Threema on the computer for a while now. At the moment, the desktop app and the web client require an active (end-to-end encrypted) connection to the mobile device. To send and receive Threema messages on the desktop even when the mobile phone is turned off, a multi-device solution is in development.
Starting today, a tech preview of Threema 2.0 for desktop is available, which allows iOS users to test the upcoming multi-device functionality ahead of time. To use the responsive new desktop application, which was redesigned from the ground up and is based on a totally new architecture, you will need the multi-device beta version of the iOS app.