Lyrics can be a great way of world-building your novel. Think "Over the Misty Mountains Cold." You can use lyrics to both develop your world and the characters who write the lyrics.
Lyrics can stand alone or they can be accompanied by music. When writing secondary-world stories, choosing a music system can be tricky. Do you want to use real-world instruments or invent your own? If you use real-world instruments, you have to make sure it would make sense for each instrument to exist within the rough time period and technological age of the story. If you invent your own, you still need to fit the instruments to other technologies and have to consider why the invented instruments were invented, what kinds of music they are used for, etc.
I personally enjoy a good lyric in a story. Whether it be a performed lyric or a more poetic lyric, lyrics are able to develop worlds like few other devices.