If you're searching for some rhyme or reason for why perpetually angry protagonist Jack whips out a smartphone-type device to play noughties-style nu-metal in the middle of an earnest moment, I'm afraid you won't find it. (Nor does the song ever come up again.) As a dark reimagining of the original Final Fantasy, Stranger of Paradise is indeed strange but it doesn't elicit surprise or wonder, just utter bafflement of how it came to be.
This can't just be boiled down to the worst protagonist created by Tetsuya Nomura in a long time. As much as the developers may cringe at the memes that have been going around since the game's reveal, it's not an exaggeration when I say most of what comes out of Jack's mouth can be summed up as "I have to kill Chaos" and "Where is Chaos?" with nary a hint of humour or charisma to compensate. I am Jack's complete lack of personality. But in his defence, his allies don't fare any better.