I will give my main answer to my question:
SBD is beneficial for merchants. Merchants are really the ones that need a 'stable' currency in order to function.
For an actual business to try and sell their goods and services (which have fixed costs to produce) they need to be able to set fixed prices. Having a stable and widely used currency will attract more merchants to sell their goods and services using the digital currency.
Merchants are beneficial to STEEM holders because "non crypto" users will have more things that they can do with their earnings from the site.
I remember trying to explain the money thing to my non crypto friends, and lots of people lost interest when they heard you have to register in an exchange, provide your bank account information, trade your SBD (or STEEM) for BTC, and then sell the BTC to get cash. I did this on behalf of many of my friends, but not everybody is going to know somebody who can do this for them.
If someone could see that they can take what they earn and go buy an i-Phone (or whatever other good/service they are interested in) in the internal marketplace using their earnings from the site - that would be huge!