Big Bad
Some developers would like to be tricky and attach content to a folder they think will exist in all sites. For instance, on most sites there's a "Templates" folder under the import node that got created by the upgrade process. So they might wish to include their templates into that folder. How they'd do this is to get the folder by URL like this:
$folder = WebGUI::Asset->newByUrl("templates");
The problem with this is that there's nothing guaranteeing the existence of such a folder. An end user could rename it or even delete it completely because there's nothing special about the folder what-so-ever.
The import node on the other hand is protected by WebGUI. It cannot be deleted by the end user, and it doesn't matter if the end user renames it, because the getImportNode method will always be able to find it regardless.