"Wouldn't it be better to have the lesson put those folders in
the already existing Content folder, and if not, what is the reason?"
The problem is: An "island A", created by Peter, can have a texture whose name
is tree2.jpg. And an "island B", created by Mary, can have a different texture
whose name is also tree2.jpg but is different. So: like a content of an
island can have the same name of that of another island, we need to relate it
to the specific island and not use the generic Content folder.