system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] So... what's the point of Daemon Primearchs?

From what I read on wiki, after achieving daemonhood they pretty much retired to warp to do daemon things for the rest of the eternity. No fighting, no leading armies, no plotting, no nothing. Wouldn't these guys be more useful on battlefield or something?
gpt
Mortarion was out and about not to long ago, he was wounded in battle against a Grey Knight recently and have been healing ever since though.

But anyway, remember that once someone ascends to Daemonhood and becomes a Daemon Prince they are no longer bound to the mortal realm and is no longer capable of being ordered around by those who may care "where they would be useful", but they are still bound to the Chaos gods who may very likely find what they are currently doing "in their retirement" to be incredibly useful, like Fulgrim, especially as those activities often involve doing things that increase the power of the god itself directly. Others are just moody and don't care in the slightest about the Long War, like Lorgar, that is something that concerns mortals only to them.