This is to appeal to Trump's religious zealot supporters.

These people are under the delusional idea that the Jews need to rebuild their temple because it has been prohecized that the Jewish temple will be rebuilt before the second coming of Jesus.

What it will do in fact is destabilize the Middle East, and create more Muslim extremist fanatics who will hate the U.S., which is something Trump has been warned about multiple times by many different persons and virtually all Middle East experts are in agreement that this will have really bad consequences. Trump, being the idiot that he is, is going to do it anyways, because he promised his fanatical Christian supporters that he would.