ReactOS is meant to be a drop-in solution. Just install it over your existing Windows installation and keep using your machine like nothing has changed. That's the goal anyway.
But I can see an obvious use case - testing. Customer reports an error but you can't reproduce it on your company's Windows development machines. I was able to reproduce a couple of bugs that way with wine and I imagine it could be handy running under ReactOS.
* I've only ever tried it in a VM.