As I understand it, the Sinaloa cartel are masters of tunneling under the border and transporting people and drugs. If this is true (wall has no effect on migration), is there any other political purpose to build a $25B wall?
Not in any way, shape or form.
The bandwidth of a tunnell is immeasurably smaller than of open terrain.
You can only transfer a few people at a time through the tunnel. You can transfer hundreds or thousands more people in open terrain.
The tunnels are possible to detect and destroy. Given the geopolitical situation, that possibility gets easier and easier with better technology
Israel was forced to develop the technology due to the threat of Hamas tunnels from Gaza. That know-how will only increase. Unlike the tunnel technology - you can't make a stealth undetectable tunnel, especially given cost constraints.
Additionally, tunnels can't be dug everywhere. So, the interception and enforcement becomes easier as you only need to cover small areas where tunneling is possible. You can't tunnel through a rock or under a river.
Speaking of cost constraints: tunnel making costs money.
The reason people get smuggled in such large numbers across the border is economics - the cost of smuggling a person (adjusted for risk of interception) is less than economic benefit of successful smuggling - which, in turn is paid off by economic benefits of a smuggled person being in USA.
Needing to build the tunnel affects both microeconomic sides of the equation - you raise the cost of transit a lot, and you reduce the amount of people being smuggled. At some point, it becomes simply uneconomical to use the tunnels to smuggle people.