The music editor working on a film or television music project is in charge of tracking and managing the different pieces of music used in the production. Also, it's quite common for the music editor to be involved with or in charge of creating the "temp music" track which is created to play along with early versions of the film before the actual music is written/chosen for the film.
This "temp" music, heard extensively at test screenings, represents opportunites for composers and songwriters to get their music heard by filmmakers and music supervisors. Even though the music may not be the actual music chosen for the film, having music as part of early versions of a film can lead to many future opportunities.
Moral of the story: for this reason, it's great to have good relationships with music editors, and to keep them supplied with your latest music for use in temp tracks they create. While having your music used on temp tracks doesn't generate any money for you (and some people might argue that it should), it does create a situation where your music is being heard with picture - and that can lead to all sorts of opportunities.