As the title asks, what is this thing for? Have never seen anything like it at other airports I've been to.
It is just a design feature, and serves no technical purpose, beyond filtering some light and providing a waterfall feature.
Safdie designed the roof of the rotunda as an inverted dome, an asymmetric dish floating above the piazza that lets sunlight in and filters the daylight with a special white fabric ceiling.
Rain falling upon the suspended dome drains towards the center and enters the rotunda through an oculus skylight as a waterfall. In the dry season, a continuous flow of water washes the roof, contributing to the rotunda’s cooling and forming a fountain through the oculus.
Image, because it looks nice (Same source as second quote above)
It's Architecture [sic], and it's for your eyes to behold:
This is the decorative roof above the duty-free area, which is circular with a plaza in its center. From the top center and around the "sun roof" there a circular waterfall.