Music for relaxation is just that, to relax the mind. Typically no thoughts of past, just to stay in the moment so the mind does not wonder. Without lyrics, there is no association of a thought or memory so you can stay in the moment. This type of music is good for sleep and relaxation.
Like we do with movies, putting on a sad song will help process sadness. If you want be motivated to work out, then listening to something upbeat will help increase activity. For sleep, it's best to listen to relaxing music and nothing upbeat. When working on feelings in therapy, you can use music to help activate your thoughts and feelings of the past to help process unresolved matters.