If you check your wallet, you'll see that there are three tipes of curruncy:
Now you may ask: "STEEM and SBD are the same, they don't do anything else other than carrying value, right?"
Well, not exactly: STEEM is a cryptocurrency, SBD is a tool
Yes, SBD is a debt-tool, it's used on STEEM-based sites; such as Steemit and DLive; and can be converted back to STEEM. It's always worth about 1$.
It was created to stabilize the economy of STEEM. Because cryptocurrencies prices (in this case, STEEM's) can fluctuate a lot, people could see their wallet value be 10$ in one day and 13$ the day after, SBD help prevent this fluctuation.
So, if you want to take risks, have STEEM in your wallet, and when it rises you'll benefit from it
Or, if you want to play safe, hold SBD and don't worry about STEEM's price