The first real example it opens with, is a criminal. For the rest of ICE detainees, don’t care. Again, we have 10s of millions of backlog of illegal entries, overstays, visa violations etc. No one knows how many 10s of millions, but it is eight figures worth. Most of the illegal entries we have essentially no way of getting or knowing who they are. Collecting their biometrics to include DNA that might be found at future crime scenes is a bare minimum. Likewise as others have pointed out collecting family members or children’s DNA is essential for dealing with rampant trafficking.
ICE are killing 1 person per week this year. They've tried to game that number down by releasing people as they're about to die from custody. This is a large well organized semi-military force with the backing of the state ($ Billions) literally terrorizing and killing people. They are clearly trafficking humans.
If I had to guess, you consume a lot of entertainment media.