I think it boils down to wise use of resources. Focus on the criminal illegals first and stop the flow of new illegals.
Deporting some people who have not committed crimes is a way of stopping the flow of illegals and the numbers at the border show that. Under Obama especially but under previous administrations the message has always been that if you can get across the border or overstay your visa then no one is going to bother you. Now you have people going back to their villages and saying "if you go, you never know when you could be deported and it is much harder to get across the border." It is the messages that go back to those thinking of coming that lowers the numbers.