Every time I go to raid a Stormcloak camp, there's always a named soldier, who is always very weak, which cannot be killed, he just kneels, coughs for a bit, then gets back up for me to "kill" him once again.

Can anyone provide an explanation as to why this is?

He's essential, that's why you aren't able to kill it.

I know, but why? What is the point to a random invincible person in the camps? From what I've seen they have no relevance to the main, civil war, or any other quest line besides just being another soldier in the camps.

Civil War quest lines

I have beaten the Imperial quest line, never did I hear anyone talk of these unkillable characters in the camps.

And you can explain in more detail, this is the spoilers forum, after all.

They're still invincible after you finish the questline on either side. That's the problem.
You're told to wipe out the camps after the questline. You can't because they'r still flagged as essential.

It's [censored] [censored].

LOL

But yes I agree.

Why tell me to wipe out the camp if I actually can't because of game mechanics?

May be they are walking camp-flag-pole? or may be... the camp will be wiped out from the map if they die?... whatever... just use your shout to push them fall off the cliff

once they walk back do it again over and over

I hate that I can't wipe out those camps :/