James Joyce wrote that proper art fixes the beholder in aesthetic arrest, and that elicits an emotional response of any kind is of a lesser quality. He said this because he's a fucking moron, and fucking morons say stupid things.
Art is powerful because it either allows us to discover things about ourselves or allows us to express those things, saying, "Yes, that's how I feel" and emptying the proverbial container.
Most of the music I listen to and identify with is sad. This is because despair and sadness are the overwhelming emotions I feel most of the time, and listening to music like "Song For Zula" is a way to get that feeling out of my system quickly and safely. Left alone with my own thoughts, I'll circle the drain for days on end, while one trip around a The National album gets enough of it out of my system that I can usually at least go to sleep and try again tomorrow.
Except that I don't know that "get it out of my system" is the right way to phrase it. It's not about expending or purging negative energy so much as descending to the absolute lowest, darkest level you can find and looking the sonofabitch in the eye, just so you know what you're dealing with.
I don't know if that's what makes "Song For Zula" great art (though I believe it is great art), but it does make "Song For Zula" an incredibly powerful piece of music.