The Ottoman chefs were the first to understand the benefits of roasting meat vertically. Doing it horizontally meant that all the yummy juices would just drain in the fire. With the vertical spit, you could have much more tender, juicier and softer meat.
With the Ottoman empire stretching all over the Middle East and Levant, the shawarma’s popularity grew, both within the region and in the West, where it was introduced by Middle Eastern immigrants.