Trump advocates protectionist economic policies for many reasons. However, the main reason why he does it is neither for domestic economic purposes nor for trade itself.
Sending a Clear Message to China
The main purpose of Trump's wall and his statements and tweets on protectionist policies is not to keep migrants out: it is to send a signal to the Chinese that Trump is willing to impose a crushing embargo on Chinese goods entering the US in retaliation for China either occupying or embargoing either Taiwan or Hong Kong.
Chinese Strategy: Aggression Until It Forces Americans to Either Capitulate or Go To War
The Chinese policy in the region is to take action that compels America to either capitulate to Chinese aggression or go to war.
Recall that when Russia annexed Crimea, the West immediately responded with sanctions and other penalties. Doing the same with China would prove far more costly domestically to Americans, which is something the Chinese believe gives them leverage over the US government.
The Chinese believe the American people would not tolerate prices jumping 200% at Wal-Mart and a huge decline in a standard of living for some obscure islands in the Asia Pacific or a massacre in Hong Kong.
Fight Chinese Blackmail with American Blackmail
Kissinger anticipated China trying to squeeze America and her Pacific allies through an embargo or other low-intensity military activity against Taiwan, Hong Kong, or other regional players, and so he believed it was crucial to have a credible non-military response to avoid giving the Chinese the initiative in any shooting war.
Chinese Embargo: Painful for Americans Like OPEC Shock in the 1973
An embargo on China will prove painful for Americans and very hard on the economy: it will be as much a burden and shock as the 1973 OPEC embargo was, if not worse.
Trump's Protectionist Rhetoric and the Mexico Wall: Prepare Americans to be Under Siege
Kissinger wanted Trump to prepare Americans to suffer through a possible embargo of Chinese goods, and so while Trump does aim to achieve some real trade improvements for the US, the foreign policy objective of Trump's protectionist rhetoric is to prepare Americans to accept an economic shock if Trump is forced to impose a retaliatory embargo on Chinese goods.
Will It Work?
An American embargo on Chinese goods would definitely put huge pressure on the Chinese elite to be more receptive to American wishes. Whether or not it works comes down to internal Chinese politics, but Kissinger knows this better than anyone.
China and Trump: Watching Each Other
It is imperative for America to be prepared to impose a credible and long-term embargo on China to avoid either capitulating or resorting to a mutually destructive war in the Pacific. Therefore, Trump must match his protectionist rhetoric with the regional aggression from the Chinese, as the Chinese clearly understand what Trump is saying when he talks protectionism about Mexico: every tweet and public protectionist statement from Trump helps psychologically prepare Americans to support a trade embargo on China as not only good for the American economy but patriotic, too.