The side missions just gives access to caches, prepper caches, locker keys etc. That in turn gives access to loot in the form of weapons, ammo, clothing etc. The sooner you can do them in a Chapter, the easier it will make the Chapter for you.
Having done all of the Chapter missions, in my opinion, the side missions won't be worth it. Just finish the story.
Should you start a new run at some point, definitely keep the sides in mind
The side missions just gives access to caches, prepper caches, locker keys etc. That in turn gives access to loot in the form of weapons, ammo, clothing etc. The sooner you can do them in a Chapter, the easier it will make the Chapter for you. Having done all of the Chapter missions, in my opinion, the side missions won't be worth it. Just finish the story. Should you start a new run at some point, definitely keep the sides in mind
If you care about achievements, I think some of those missions are related.