It's an incentive, from my understanding.
As an incentive you put up tiers as goals for your readers to reach and then give them rewards of each tier as they pass the threshold. This encourages people to earn FREE INK and to BUY INK to get more of the thing they want, so it helps promote the event. But it's all an incentive.
If you do not want to make a reward, or only want 1 reward, then that's it. You aren't "required" to participate in the event even if it's site-wide. I'd suggest just jotting down ideas for your characters as an art piece as a reward like: "if I hit 1000 ink then you'll get to see this artwork for my characters messing with a chocolate fountain".