According to the tenets of Islam, the Kaaba (above) was constructed by Abraham as the first house of worship to Allah. It therefore is the holiest spot of the Islamic faith, and all Muslims are expected to visit it at least once in their lives (the Hajj).
When combined with other locations of immense cultural significance elsewhere in the city of Mecca, the Kaaba grants the ruling House of Saud immense cultural and political power. That power extends far beyond Saudi Arabian borders, stretching throughout the entirety of the Muslim world.
While the United States retrenches from the world at large and the Middle East in specific, the Saudis are using this religious authority to reshape their neighborhood. This includes generating new militant forces to fight the Saudis’ battles in places like Chechnya, Afghanistan, Pakistan, Iraq, Syria, and ultimately Iran. It is an ugly, bloody strategy that leads to groups like al Qaeda and ISIS. But it is damnably effective.
For more on the Saudi strategy for managing its neighborhood in the world after free trade, see Chapter 9 of The Accidental Superpower.