The objective of this diff is to allow changing the directory contents of the icons folder without having to maintain a file dictionary in the codebase, similar to including an entire directory of JS or XML files.
One actual use case are country icons in the network dialog 3787 / D1746, and possibly the lobby list.
A second actual use case is loading the resource icons folder, so that mods don't have to overwrite the setup_resources.xml file.
That would also allow combining mods that both change resources.