I watched the sci-fi film Silent Running recently and noticed that the film had many songs written specifically for the movie. Was there a specific reason as to why this is so? Was it common amongst other films of the time, or was it specific to this movie?
In the extras on the blu-ray of Silent Running, director Trumbull talks about the great difficulty he had in recruiting Joan Baez to do the title song (and a lovely song at that). Baez wrote the song.
Why did he do it? It adds immeasurably to the mood of the film, a fragile innocence, especially when interspersed with the somewhat nationalistic themes played for the exterior shots. This changing of music enhances the 'environment versus big government' theme of the film.
Getting that effect from existing music would have been far more difficult than having music tailored to bring out that contrast.
From a different perspective I'm going to suggest the reason was more mundane : money.
- purchase rights to use existing songs in your new movie
- result : money goes out the door in the form of the initial cost and/or ongoing fees.
- pay to have songs written for your movie
- result : initial fee, but you get to keep any and all revenue (or some slice of it) from later e.g. radio plays, record sales and performances.
I don't know about you, but wearing my Imaginary Film Producer Hat I'd prefer the B option. :-)
From an artistic point of view there was a desire to have music that reflected the movies themes and Joan Baez was, as it were, the Man for that job. :-)
It's just a matter of what kind of risk-reward model you want for your business after that.
An example of other films that have at least a song directly written for them are the Bond films, most of which have had their title songs written specifically for the movie.
Flash Gordon had it's soundtrack written by Queen, so the idea is not without precedent (though you could argue that Flash Gordon was an anomaly and not the norm).
If you broaden the scope of "song" to include any musical performance, then the majority of films have a score composed for them specifically. If you're limiting it to song in a more conventional sense, film musicals are another example.