If you're illegal and not sure if you'll be rounded up you're more likely to act in ways that indicate a quick decision to relocate. Wiring money, plane ticket purchases, etc.
So it could be a way to flush out those flying under the radar and or who have managed to become invisible to the court system. Make the notice specific and short notice to ensure panic induced transactions.
ICE can Just monitor electronic transactions for suspect behavior related to fear of being rounded up. That's my guess.