In the early days of Vinyl, the main purpose of a cover or sleeve was to simply inform the consumer what the record consists of, this may typically include a picture of the artist followed by the artist's stage name and the name of the song provided on the record. The CD cover is nowadays a representative of who the artist is and their style. In modern times anyone can judge an artist based on the CD cover, for instance if a cover shows a close up of an artist's face then it demonstrates that their music is more about promoting themselves as a performer because they want people to recognise them with their face. it would be most unusual if a recording artist had a picture of their face on the cover but their music was all instrumental. If on the other hand a CD cover has no picture of the artist atall then it demonstrates that the artist is more into promoting their music as opposed to their image.
Contemporary pop music has began to demonstate a more surrealist nature as demonstrated here on Lady Gaga's album 'The Fame Monster' the purpose of this cover is to promote the style of the artist rather than the artist themselves.