But why?

Sure, there are people who want to tinker with it for fun, but outside of that what is the point in having Swift on Linux and Windows?

You can't use it to build mac or iOS apps on Linux because the important libraries aren't there. There is no cross platform UI and Apple isn't likely to port and support theirs from macOS. If you are into servers then you have plenty of better languages options with established ecosystems. Maybe there are some command line apps on macOS which can now be ported to Linux/Window, but beyond that, what is the point? Where is Apple trying to go with this?