It's the parallel collection, for comparison purposes only. Some of these songs are extremely rare or obscure, and others are common as dirt. The idea here is to provide a quick and easy means of accessing them when listening to or reading about the project.
The folder are a bit larger than the ones for the covers, and I didn't break these down into CD-sized batches as I did with the project songs. This shouldn't cause any trouble other than that these will take a little longer to download.