There are a few differences between Steem and Steem backed dollars
STEEM is liquid currency that you use to POWER UP (Vests) and you can't Power Up with SBD.
STEEM also becomes diluted over time due to its 100% yearly inflation rate and it's basically like holding shares to a company so it is not designed to "HODL" but to use as a means of transferring value (or shares) as well as powering up. (When you use 10 STEEM to Power Up today you get X amount of Vests. Next year to get the same amount of Vests you got today for 10 STEEM will cost 20 STEEM.) There are a few articles that explain that this is done because there's no "company" to pay you "dividends" when the "share pool" is diluted.
Before SBD was available in the exchanges, you had to trade SBD for STEEM in order to play the market with a "Steemit" coin. Now you can do that with both.
SBD is also supposed to be worth about $1 USD so that it would be easier to make transactions on a day to day basis (kind of like a tether) without having to calculate the current value of the coin. Obviously this has changed recently! SBD doesn't lose value over-time like STEEM does you can hold your SBD (pun intended)
I also read that SBD is more like the internal currency. Meaning you can't exactly "Invest" into the "company" with SBD, but use them as a currency.
Last month and in november most of the liquid STEEM that were generated were used to power up.