I'm sorry to hear it's so much at once. Each of those is a lot to deal with.

For what possible reason are they using torture on people arrested for immigration offenses?

In Abu Ghraib, they tried to say they were after information about future terrorist attacks. Stupid and monstrous, but they had a purported 'reason'.

What the fuck is this supposed to be for?

And the tortured have never been convicted of a crime, and the worst thing most if not all of them did was come to the US to make a better life for themselves, sometimes traveling by foot for hundreds or thousands of miles.