It's a safety mechanism.
The 'base feed price' is used for sbd conversions. If the price of steem is very low (which is is right now) and people convert their SBD, a lot of new STEEM would be created. This safety mechanism prevents that.

From the Steem whitepaper:

A rapid change in the value of STEEM can dramatically change the debt-to-ownership ratio. The blockchain prevents the debt-to-ownership ratio from getting too high, by reducing the amount of STEEM awarded through SBD conversions if the debt level were to exceed 10%. If the amount of SBD debt ever
10 of 32 exceeds 10% of the total STEEM market cap, the blockchain will automatically reduce the amount of STEEM generated through conversions to a maximum of 10% of the market cap. This ensures that the
blockchain will never have higher than a 10% debt-to-ownership ratio.

More info about the SBD:
https://steemit.com/sbd/@timcliff/sbd-explained