The following text is from the wiki:
Atlas and his brother Menoetius sided with the Titans in their war
against the Olympians, the Titanomachy. When the Titans were defeated,
many of them (including Menoetius) were confined to Tartarus, but Zeus
condemned Atlas to stand at the western edge of Gaia (the Earth)
and hold up The Heavens on his shoulders, to prevent the two
from resuming their primordial embrace. Thus, he was Atlas Telamon,
"enduring Atlas," and became a doublet of Coeus, the embodiment of the
celestial axis around which the heavens revolve.
As I understand it:
a) In a way, its the moment in creation, that the earth is separated from the heavens (or the opposite). The punishment inflicted by Zeus, for the punished Titans to live in the world of mortals.
b) Atlas holds the celestial sphere to separate Gaia(Earth) from Uranus(Sky). If they united together once more, Gaia might start birthing titans again and the Olympian Gods see that as bad thing.
In both scenarios, they were not separated before Atlas.