There is a story that ties the campaign together, mostly acted out through dialogue trees and (so far optional) mission objectives.
The missions are tied together through control of the overworld map. For example, the first fortified position that you are supposed to take has some side quests tied to it that make it’s capture easier. One of these is to stop the fort from buying explosives from a vendor in the area. You can try to talk the vendor out of selling to the Legion or you can kill her; both accomplish the task.
There are morale systems for friendly settlements, too. If you’re working to improve the local conditions, then your mine output is better.
It’s pretty close to the mission/downtime mix from something like XCOM.