What exactly is a perpendicular Universe?

In Old Man's War what exactly is the point of fighting when each time one travels they are in a parallel universe?

• It just seemed so pointless to me, if you were fighting to protect your universe A, you'd then travel to universe B, fight that B version, travel back home to universe C where everyone is apparently the same but not really, just a C version of them. You leave to protect spouse A but come home to spouse C.  Furthermore, what would prevent you in universe C from being there already?  What forces would ensure that your C is now in another parallel universe, etc, etc, etc?  Really enjoyed the novel but couldn't quite wrap my head around that part and it made my brain hurt. Like what if you went to universe C and your other people from B didn't got to C and then move on to D so it's like nothing is gained because your victory in B doesn't matter because that victory never happened in your final destination of C? My head hurts just thinking about it.

• Answer:

  It really has to do with people's personal perceptions. If you don't know how the skip drive works, you'd never know spouse A, B and C are not the same person. Effectively, they aren't. But the biggest reason is that if you do nothing, you have to deal with the issues that are the reason we are fighting in the first place, so you need to go to battle in the parallel universe to make a parallel universe where you have solved those issues.