Doctor Who might be a family orientated show with a lot of light moments, but occassionaly the show can go dark.
During the Tenth Doctor's run, The Doctor has a moment where he thinks he is a god. He thinks he is far superioir to every other being in the universe. This is also the moment, when the Tenth Doctor believes he has lived too long, and that his run should come to an end. I consider this moment to be one of the darkest moments in Doctor Who.
This musical piece just creates the perfect setting for this moment, and it expresses it perfectly.