They weigh how markets second guess their future actions, and the implications thereof. Most importantly, they employ financial markets to transmit policy impulses from the short end of the market spectrum to the longer end and from there to the rest of the economy.
In fact, financial markets produce outcomes which are akin to public goods – they involve externalities in that they benefit all without incurring any costs; and they are nonrival in their consumption -– they do not exclude any from their benefits. I would hesitate, however, to classify any market outcome as a public good for fear of being charged with heresy.