To keep everyone else in line. Americans are so programmed to defer to aw enforcement that they will watch the most blatant abuses carried out right in front of them with little other than hand-wringing. Immigration status is just the excuse, compliance is the goal. What do you think is going to happen at the next election? ICE doesn't even need to intimidate people at polling places, just the rumor that hey are doing so will be enough to scare many citizens away from voting in person. They could vote by mail, but no doubt you're aware that the President ad his party constantly impugn the validity of such votes. How much do you trust them to uphold and abide by the voting process? We've already seen what happens when they get a result that's not favorable to them.