On the one hand, Rand Paul and other GOP Senators have a Top Goal of Reducing Government Wasteful Spending, but the Biggest Gov't Spending is Welfare for Illegals and Fighting Crime & Drug Addiction caused by Illegals.
Hence, you can't do these effectively (or efficiently) without a Southern Border Wall.
Moreover, Senator Rand Paul and other senators say building a Border Wall via Executive Order is Executive Overreach. Yet how is that executive overreach as the President, as well as Congress, is supposed to protect America?
Without a Southern Border Wall, the little problems like illegal border crossings and drug trafficking can and does become big problems like Welfare Waste & Fraud, overcrowded jails, and overloaded courts due to illegals.
Since, Senator Rand Paul is also a doctor, he should know what "preventative medicine" is and that it is medical practices that addresses the little problems so that they don't grow to become big problems. For example, early detection of cancer is "preventative medicine".
Accordingly with Southern Border Wall, it stops (or significantly reduces) the little problems like illegal border crossings and drug trafficking that become big problems like wasteful government spending on Welfare for Illegals and Crime.