There are two ways for Movies to go. They either have a Score or a Soundtrack, or sometimes both. Usually the scores are forgettable and the soundtracks filled with either 40 year old Thin Lizzy and AC/DC songs or new songs from bands no one will remember in 10 years let alone 40.
Occasionally however a movie will make tremendous use of a song. Every time you hear it from then on you will remember it. My own example of this is the way that Nine Inch Nail's The Mark has been made, was used in Man on Fire. Every time the percussion starts in the song I can picture Denzel Washington going and picking out all the supplies he needs to wage a one man war on the Mexican cartel that has kidnapped the little girl he was supposed to protect.
It is an ominous and haunting song on its own. But when you add in the pathos of the scene where a still bleeding John Creasy is preparing to get revenge, it becomes almost depraved through its anguish. The music accentuates it to a near feverish pitch.
Of course, this is just my opinion, so your mileage will vary.