The Bastions were an important part of all the buildings, but as a resident of the Citadel, Adam did not have the opportunity to fully understand their importance.
In his case, he would only fully realize the situation once he became strong enough. If he could become a Second Rank Phantom in the near future, he, like every Phantom of that rank, would be given a specific role.
Sure, it's unlikely that the government would send him to defend a Bastion, as all the positions were already taken, and he wouldn't be strong enough for that as a newcomer among the Second Rank Phantoms.
But the Citadel and the Bastions were points of extreme importance. Humanity couldn't afford to lose any of them, because like the Citadel, the Bastions were necessary for a quick recovery after the World Wipe.
The faster humanity could recover, the fewer losses there would be from the monsters, which had adapted much faster due to their numbers and energy.
