Well, say all you've got is an x86 device, but you want to develop for ARM. You can write and compile your code, push it to unicorn, and see how it runs.

Or you can use it as a sandbox serving x86 software on an x86 machine.

Or as a "virtual machine" serving say AOSP for ARM on a Windows x86 host.

There's a long list of projects using Unicorn at https://www.unicorn-engine.org/showcase/

How's this one differ from QEMU?