Well, witnesses can affect SBD price in 2 major ways:

1: increased supply which should decrease it's price, through their pricefeed.
(Check steemian.info/witnesses to see each witness' price broadcast)

2: the top 20 witnesses are in charge of accepting or rejecting a hardfork, if 17+ of the top 20 start using the new version, than the rest of the witnesses will have to follow through.

Bonus example: The witnesses also set the interest rate on holding SBD, in case SBD falls too low, they can increase is from 0% to motivate people to HODL it.

Also witnesses are Steem's version of mining, using delegated proof of stake. Where votes for witnesses increase their chance of mining a block, similarly like more GPUs increase the chance of an ether miner to find a block.