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@halsadi
Plain and simple: inflation.
Basically what happens is every 3 seconds a block is added to the Steem blockchain, and with (if I'm not mistaken) 2 Steem units are added to the reward pool, as more content gets upvoted the reward pool gets depleted, and as more block are added the bigger the reward pool gets. It's basically a question of which of those procceses happens faster.
You can track the reward pool on steemd.com to see how it fluctuates over time

THEY CREATE THE MONEY!
And because it has a demand - It has a market value.