there is one obvious answer to all and that is power, the more deeds you do for your god or chaos you stronger you become.
indeed nurgle gifts doesnt sound all to nice do they but keep in mind living in the Chaos wastes isnt exactly a sane place. Then of course you get very tough when given the mark of nurgle and that should come in handy for many warlords.
Slaanesh is obvious, his followers are utterly devoted to pleasure and pain. the more excessive it gets the more attention you get from your god. As for the mark you no longer are concerned by fear or such but you presence become inspiring and alluring, quite effective for a chaos general wouldnt you say?
exists on alot of places and alot of levels, who wouldnt want to appear intoxicating and inspiring to others?
Khorne doesnt really need much explaination. Kill = get rewarded, kill alot = get plenty of rewads.
doesnt really have a sect, he is worshiped on the battlefield.
Tzeentch, power power power power. gain knowledge of the future, manipulate the future.
the sects are very secretive and selective and only the leader of the cult knows the identity of all members.
The gifts, like tentacles, fangs and other mutations might not sound all to good to us but to them it is a sign they are touched by the gods and that leads to status and respect.
the end of this road is Daemonhood which is the reason anyone of these champions start their journey in the first place, to join his god and reach immortality.
then if you wander off getting too many rewards you end up as a spawn but that is a different matter
