Forecasts made under such an assumption should better reveal the participants' views of the underlying structural parameters of the economy – the best possible tradeoffs between inflation and output stability in the short run and longer-term price and output trends – than would be made assuming an externally derived path for rates, such as the path implicit in market interest rates.
If the markets' path for appropriate policy were at odds with my judgment about appropriate policy, I would have to write down a forecast whose path or end points were not consistent with what I believed were our best efforts to achieve our objectives.