Pinky
and The Brain...
Pinky - look at a practical (recent) court example: Spirit/estate of Randy California vs Led Zeppelin. Both pieces of music were copyrighted. Spirit's song was first. The court decided (or would have it was not settled out of court? I'm not even sure if this is still resolved) based on 'similarities' and the fact that the Spirit song (copyright) came first.
If Spirit/California had not copyrighted the song, Led Zep could have said they wrote it first and demoed it, which California heard and copied, and Zep would have won in court.
Or worse, Zep sues them for recording the song year(s?) before.