What is a primary use of a USB key in service technician tasks?

The primary use of a USB key in service technician tasks is as a bootable diagnostics tool. This function allows technicians to load diagnostic software and tools directly from the USB key to troubleshoot and repair systems. When a USB key is made bootable, it can contain operating systems or diagnostic programs that can run independently of the installed operating system on a device. This capability is invaluable when dealing with a computer that won't boot normally or that requires technical diagnostics.

While other uses of a USB key, such as data storage, system backups, and data encryption, are certainly important, they are not as crucial in the immediate context of service and technical diagnostics. Bootable diagnostics often play a critical role in troubleshooting hardware and software issues, making this particular use case stand out among the options provided.