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steem is a volatile asset.
the price is hardly settled on supply and demand on the markets. (atm ~13 cents).
steem dollar in comparsion is a stable currency which normally should be around 1$ Us dollar.
they are very useful if you have worries about volatility. (/loosing some of your money)

so steem is the caponent you use to speculate and then cash out to steem dollars with gain intrest is that correct?

this could be one way to look at it.
you are right, steem would be the tool to speculate. but the interest rate of SBD isnt that profitable.

maybe this will help.
http://steemwiki.com/index.php?action=artikel&cat=1&id=12&artlang=en

15percent apr sounds like a lot to me as long as my dollar stays worth a dollar