"Banks are optimally opaque institutions. They produce debt for use as a transaction medium (bank money), which requires that information about the backing assets – loans – not be revealed, so that bank money does not fluctuate in value, reducing the efficiency of trade."

-Banks As Secret Keepers: https://www.aeaweb.org/aea/2015conference/program/retrieve.p...

There's a recent but very influential paper that shows that banks need to keep their assets secret to function well.