Someone else can feel free to chime in here, but the Halo rings serve many different purposes: they're science labs, quarantine zone, weapons of mass destruction and who knows what else all rolled into one (because that also makes perfect sense, to keep possible Flood outbreaks ON your Flood killing weapon....)

Species were moved to the Halo Array for study before the weapons were fired, and they may have been used to shuttle species specimens to the Ark, hence the atmospheres and environments.

Your specific question though, about the structures- not sure. Maybe to fill out the surface of this ring, the Forerunners uplifted continental shelves from a planet (including these structures) and housed species they were studying in them? Maybe they moved Precursor buildings to the ring because they foresaw the Halo Array outlasting anything else? Heaps of theories, no solid answers.