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'Cause you're using it in the background as you're voting to get your SMTs. You're getting your SMT token, plus some Steem as a bonus. The two systems work in parallel. Like with @steemhunt, you get your HUNT tokens and some Steem for your upvotes. However Steem isn't the motivator for upvoting, HUNT tokens are.

Why would you need motivation for downvoting? Each dapp can have their own rules for their community alongside with their SMTs. If someone disregards the TOS you can exempt them from earning SMTs. It incentivizes good behavior. The people running the dapp are basically mods for their dapp. You need some sort of centralization to keep bad actors in check. Steemit.com is and should be a place where anything goes no holds barred. But as you can see you can't really do anything if someone goes wild in that system. Flagging doesn't help at all. Still it's nice to have a place like that existing for people who crave absolute freedom.

I'm giving up, I'm sorry, I don't know how else I can make you understand. I hope someone else will come along who can explain things better than I can.