In every loot distribution system, there's a way to "earn" points, or more generally, to increase one's probability to receive future loot. When considering rewards, it is most important to be aware what exactly is rewarded. Zero Sum systems in their pure form for example only reward presence at the moment of loot distribution. If on a raid no (or very little) loot was distributed because it was a learning run for a new boss encounter, it doesn't feel fair that the members should go empty handed. Common reasons to hand out DKP include: