The Bastion was constructed to serve as a sanctuary for displaced citizens in the event of an emergency. The Kid sets out for the Bastion after the Calamity, hoping to find more survivors, and invites survivors that he finds during his travels back there. At the start of the game, the Cores which power the Bastion are inexplicably drained and must be replaced with new ones scavenged from crumbling locations outside the Bastion.
The Monument may also activate an Evacuation protocol which destroyed the Cores in the process.
Rucks designed the Bastion himself, which is why he is intimately familiar with its workings and true functions.