The Steem Backed Dollar has a value linked in Steemit to 1 dollar, although they will not give you 1 dollar really but rather they will give you their equivalent in Steem. If the price of the Steem Dollar remains stable with the USD, then there is not much action, but if its price increases above its "real" value, then we are earning an extra for every dollar we receive.
With this extra we can, for example, send the SBD to an exchange and change it there by Steem, to then bring the Steem back to the platform and do power up, increasing the SP even more than we would have gained if we simply made power up in the publication.
If the SBD has a value of $4 in the market, then it is similar to that they were giving you $4 to buy you only $1.