Lyrics are just a reason for the singer to make sounds. Since lyrical content is as such arbitrary, you may as well make them as creative as possible. As such, songs are not just poetry sung over music, it's a way for the singer to tell a story or concept to us. When was the last time you read a really good book that rhymed all the way through? =/
(This spot reserved for extra flavor for those that have nothing interesting to say in their posts)