Why does Socrates consider philosophy to be the greatest music?

In Plato's Phaedo Socrates says

"The same dream came to me often in my past life, sometimes in one form and sometimes in another, but always saying the same thing: 'Socrates,' it said, 'make music and work at it.' And I formerly thought it was urging and encouraging me to do what I was doing already and that just as people encourage runners by cheering, so the dream was encouraging me to do what I was doing, that is, to make music, because philosophy was the greatest kind of music and I was working at that. But now, after the trial and while the festival of the god delayed my execution, I thought, in case the repeated dream really meant to tell me to make this which is ordinarily called music, I ought to do so and not to disobey."