Music Publishers are interested about the copyright in the composition of the music. They just sign up writers, and songs. They collect the money and help the artist with the advertising and the Tv programms.
Copyright are the exclusive rights of the creator of the piece of music, of the original work of the song/the track. It includes the right of copy and to distributes the original work.
The Copyright last fifty years from the date of the recording of the original work.
Copyright is not the same around the world so an artist has to check the copyrights law and standards.
There is also the Performing Rights, it is the concept that pays the original songwriter for the use of their creation in a live scene.
Generally, the Music Publisher pays in advance the writer.
The Music publisher collects the monies of the writer for the use of their music on Tv Shows/Programs and movies, to the public, to the advertising and the performances.
PRS ( the performing rights society ) collect the monies of the writer in case of performances and reproductions of their music.
People who want to use music in a public area, need a licence from PRS.