Here is a video produced by APEM and its partner CMPA, helped by SOCAN Fundation, that explains the role of a music publisher
Music publishers are professionals in copyright management and career development for songwriters. We often hear that a music publisher is to a songwriter and his works what a manager is to a performer. Publishing activities can generate revenues in any and all contexts where music is used (recorded, covered by other artists, used in films, TV shows, advertisements, video games, played on the radio, in concert, on the web, etc.)
Music publishers represent existing repertoires and can also work to help create custom music. Partners of songwriters, music publishers play a structural role by generating revenues and by giving artistic, legal and commercial advices.