The Sun is a spherical light bulb that provides light and energy to the majority of the World.
The Sun is the messiah's tie to the world. As long as the sun is not shattered, Niko remains trapped in the world of Oneshot. If the sun is shattered, the world will end instantly, but Niko can return home.
Even if it is not at the top of the Tower, its general purpose still works as proven multiple times by Niko in the game. It can power the solar battery found in the barrens in an instant, and proves to be a functional source of light.