If you are a bank or even a money market fund, you probably want your financials to show the maximum amount of your overnight liquidity placed with the Fed’s RRP rather than with other banks. Your balance sheet looks less “risky” this way. And since most financial reporting is done at quarter end (with mid-year and year-end being the most important dates), you want to place your cash with the Fed on the last day of the quarter for one night and then take it out.