The reason is that Navision actually can only run in Client/Server mode, there is no “local” version. So when you start fin.exe and open a database directly, it launches the Server first, and connects to the database via the server, but does not expose it via TCP but uses an internal connection. This means that once you have opened a database you can open it again multiple times with the same instance of fin.exe. But you can’t open more than one database with one fin.exe, because even though you have launched a new instance of fin.exe, you are still calling the same instance of the server.
Theoretically this means that running local or via server should be identical, and thus makes for a much more reliable development environment.