It's a physical barrier that must be defeated. The deterent value is where it really pays dividends.
Now, the success rate of a Mexican wanting to illegally enter the US is most likely in the high 90s. For this high of a success rate, Mexicans are willing to spend their entire life savings and the health of their family to make the journey.
With a physical wall, the success rate would be 10% at best and that is being generous. With the odds so stacked against them, how many are going to take such a risk?
I'm not against a physical barrier, but no reason to believe an unmanned physical barrier would stop 90% It would be circumvented.