Sometimes the Cardiff Philharmonic Orchestra, made up of professionals and amateurs I believe, perform a concert of music from films. I have been to a few of these concerts but I find them, usually, unsatisfying. Some of the pieces have been made by composers or arrangers into quite good works in themselves: they have the structures of tone poems in that they unify around a theme. But often the music is slight.
Music for films early on was used to act as a dramatic accompaniment to action so that in a mystery you would expect sounds which enhanced the mystery; for comic scenes you would probably have "comic like music", light of touch, breezy. And so on.
A good deal of this music was simply sounds that of course were musical but which underlined the main action going on; the action and dialogue were there to be seen and heard, not the background music.
One of the worst examples I have seen and heard was in Hitchcock's "Strangers on a Train" in which the composer (Dimitri Tiomkin maybe?) underlines practically every piece of action with what he thought was suitable musical sound. I don't think the score of that film would make an enjoyable work to be played on its own.
But some can: John Williams's can; so can Elmer Bernstein's; so can, of course, Arthur Bliss's as for "Things to Come".
Two examples of music used most successfully in films are two in which already well-known compositions by famous composers were appropriated: "Death in Venice" with Mahler's 5th. and "Brief Encounter" with Rachmaninoff's 2nd Piano Concerto.
I think film music is most successful when you are not aware of it or, when you are, when it adds excitement to the drama: "The Magnificent Seven"; "Shane"; "The Horse Soldiers"; "City Lights".