Ahead of the Crisis
Before October 2008, the expenses and great things about keeping reserves had been clear. The fee included foregone interest, additionally the advantages included guarding against last-minute outflows that needed instant cash, much as a depositor might put aside cash to pay for crisis costs, or an investor might hold reserves allowing him to seize an opportunity that is unforeseen. In cases where a bank did require extra funds, it might get reserves via a over night loan in the federal funds market, where banking institutions with additional reserves provide to many other banks. The essential difference between exactly what a bank could provide and exactly just exactly what it might borrow represented the benefit of keeping a book asset versus the chance price of lending it down.
The total level of reserves into the banking system ended up being set because of the Federal Reserve, largely through open-market operations that provided and withdrew reserves through the market, so that you can support the federal funds price.