Track 2: Piggy
"Piggy" shows the protagonist hurt, uncertain, broken and whispering. Here he is vulnerable, almost pleading, and at the same time, makes up his mind (although unconvincingly) to let go of what he is attached to, what makes him vulnerable. The mantra of "Nothing can stop me now because I don't care anymore" is repeated softly, as if Reznor is trying to convince himself that he doesn't care.
How does the story of Jack start out? He is weak, suffering from chronic insomnia, and thoughtlessly living his life according to standards he doesn't really care about. I can very easily see the song "Piggy" representing his view of his absentee, "franchise"-starting father. In one scene where Jack and Tyler discuss who they would fight, Jack picks his boss, and Tyler picks his dad. His (their?) father abandoned him at an early age, and only gave advice a sentence at a time when his son would hunt him down and ask "Dad, now what?" during various milestones of life.
Compare that to the lyrics "Hey Pig, nothing's turning out the way I planned. Hey Pig, there's a lot of things I hoped you could help me understand. What am I supposed to do?"
Track 3: Heresy
"Heresy" is a scathing denouncement of God and religion: an aggressive stance in contrast with the previously shown weakness of the host mind. It represents the thrill of delving into subversive behavior and defying the expectations of a polite public. The verses accuse God of various atrocities in the world, and the chorus screams, "God is dead, and no one cares. If there is a hell, I'll see you there."
This might cause some among you to say, "Aha! This song has nothing to do with Fight Club, therefore the theory is absolute rubbish!" And while I would applaud you for your bold choice of using the word "rubbish" in a sentence, I would remind you of Tyler's words during the chemical burn scene: "Our fathers were our models for God. If our fathers bailed, what does it tell you about God? You have to consider the possibility that God does not like you. He never wanted you. In all probability He hates you. This is not the worst thing that can happen...We don't need Him! Fuck damnation, man! Fuck redemption! We are the unwanted children of God? So be it!"
Removing God from the throne was a necessary step in Tyler's quest for Jack to hit bottom.
That being said, the song also fits quite well in the timeline of the film. After visiting his doctor for sleeping pills and complaining of narcolepsy, Jack begins to attend support group meetings every night of the week -- by pretending to have a variety of diseases he did not have he is defying the self-regulating behavior that is ingrained in all members of a God-fearing society. It is at this point that Tyler begins to emerge when Jack supposedly sleeps, buying the house on Paper Street, making soap, learning about napalm, and working his many night jobs.