It's a fair point, why do we have a second currency?
Originally the idea by Dan Larimer, creator of the Steem Blockchain that eventually morphed into Hive, was to create a stable coin backed by Steem, hence the name Steem Backed Dollar.
This soon became apparent that there was no such backing and the value of the SBD (HBD) was arbitrary and for a while it did hold at around $1, but because there is no actual system beyond wishful thinking, to make it stay at that price it can go up or down just like any other crypto.
Like I say, the main function of the HBD should be to reward creators and add liquidity to various Hive markets.
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