Yeh QT doesn’t just use any old music in his films. The songs he uses most of the time are very relevant to the scene. In the instances where the song has nothing to do with the scene or the characters, it is usually a song/music that fits into the scene very well. Also, he times his music and what is going on on-screen perfectly. On most occasions what is happening on screen matches the music and it is (almost) always pin point correct. One example I can think of is in Jackie Brown where some music is played before Jackie gets to the mall, QT timed it so that the moment she entered the mall and started becoming familiar with the room the music changed at that exact point which matches the atmosphere at the time (see Special Edition Trivia Track).
He is like Leone in this way as Leone also used some great music at the most appropriate times in his films to great effect.
