Tri Vi Dang, Gary Gorton, Bengt Holmstrom. and Guillermo Ordonez write,

banks produce private money because they can keep the information that they produce about backing assets secret. By being opaque, banks can produce bank money more efficiently. Opacity makes it prohibitively costly for an expert investor to find out information about the details of the bank’s balance sheet, eliminating the expert’s informational advantage. Opacity also mutes the effects that public information may have on the value of the bank’s assets. By keeping information symmetric among traders, opacity makes trading in bank money liquid.

All of this was explained in chapter 6 of From Poverty to Prosperity, the book that I wrote with Nick Schulz in 2009 (later re-issued as Hidden Wealth). On p. 224-225, we write

the assumption of complete transparency, in which any individual knows all of the risk being taken by everyone in the economy, is not merely unrealistic, it assumes away the reason for financial intermediation in the first place. . .a lack of complete transparency is built into the basic function of financial intermediation.

In my admittedly biased opinion, the narrative description in that chapter is much richer than the “model” that the four authors (including the most recent Nobel Prize winner) develop. Information about risks is not a binary phenomenon in which it is either public or not. People and the institutions that employ them have different access to information and different skills in processing it. That is what makes financial intermediation in the real complex–more complex than any model can capture.