So what is the purpose of the machine?

[Alardem]

The Machine is a massive sacrificial altar - Mandus believed that in order to avert the coming horrors of the 20th century, he would need to create a mechanical god fueled by the blood of countless humans. The 'machine for pigs' is really a metaphor for his perception of the world as a grinder of human flesh.

There's many purposes to this eldritch monstrosity. It turns the rich and poor alike into meat for humans to consume, it melds pig and human together into miserable abominations, and it will 'spin the world wheel on the path to redemption'. I suspect that if the entirety of London had been fed to this monstrosity, the world created by these sacrifices would be far more horrific than our own history.