A long time ago I believe someone mentioned the protection for a song is different when the song is part of a copyrighted collection versus having individual songs copyrighted but I don't remember the difference or the example that was given.
How I've always understood that was as follows.
You copyright a collection to save money and lump as many songs as you can under the umbrella of a copyright. When and if a song in that collection is to be recorded by anyone for commercial release you copyright that one song by itself to give it it's own copyright coverage.
It's all protected with either way but there is less confusion with the individual copyright since it's under it's own name and title.