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It is explained in the Steem Whitepaper

Starting on page 5:

p5

I'm still learning, but it's my understanding that you get rewarded for up voting "good" posts, ones that eventually make it to the front page/trending. Not sure if you are punished for spam voting, but it would make sense to have that type of check in place.

I haven't come across such a check in my search for an answer yet.