Thanks, WizarDave!
The price feed is needed for the "peg" of SBD to $1USD. 1 SBD is by design meant to be worth $1 USD, this is how it is treated internally, so the task for witnesses is to fetch the STEEM market price to maintain this system peg.
In real life, SBD is not pegged to USD, but currently worth more than $2. This is why some of the calculations are a bit weird, like the big loss in overall return if you go for 100% powerup compared to 50/50 payout.
Please let me know if you have some more questions or if the explanation was not clear. :)