The integration with RustDesk is the same like VNC mRemoteNG integration. I think that the porpose of the mRemoteNG is to centralize all remote connections ...
mRemoteNG adds bug fixes and new features to mRemote and allows you to view all of your remote connections in a simple yet powerful tabbed interface. mRemoteNG ...
The list below of various examples is by no means a full list of ways to use External Tools but gives you a idea of how it can be used in different ways.