...because Visual Studio is a C#/C++ IDE that works on Windows, and Xamarin Studio is also a C#/C++ IDE that works on Windows?
Xamarin Studio is a swiss army knife. I don't know about your use case, but I use it under the name Monodevelop on linux (not really sure what that association is) and as Xamarin studio on windows. For my use case, it's a C# and C++ IDE for desktop development, same as visual studio.
It's certainly not a MacOS only product, it supports all three platforms including windows.