.NET is one of two development platforms you can choose to build Remoting SDK servers.
You have your choice of C#, Swift, Oxygene, and Visual Basic as your programming language (the latter in Visual Studio only, the other three available in both Visual Studio and Fire).
In particular, Mac and iOS developers will find the combination of Fire and our free Swift compiler for .NET ("Silver") an attractive option for adding a server-side component to their Cocoa client app.
Servers built in .NET can be hosted natively on Windows, and – more excitingly, for live deployment – you can use Mono (and soon .NET Core) to host them on Linux Servers, and also on Mac OS X.
The nice part is that, when written with some minimal care about cross-platform differences, the same server executable can be run on all platforms. This makes it easy to, for example, test and debug your server as you write it on Windows or Mac, and then push it to your Linux Servers for live deployment.
.NET based servers are really easy to deploy to cloud-based hosting solutions, such as Amazon Web Services (via EC2) or to Microsoft's Azure. Of course you can also host them on your own dedicated or co-hosted server hardware.
Check out more details on this in the Deployment section.