A bunch of money market funds and other cash investors keep $1.8 billion of cash at JPMorgan and Bank of New York Mellon, the “clearing banks” in the triparty system.
A bunch of securities dealers keep a pile of securities – worth, on a good day, more than $1.8bn – to JPM and BoNY Mellon.
The dealers need money to fund those securities, because what are they going to do, pay for them themselves?
Every afternoon, the cash investors and the securities dealers frantically negotiate which dealers swap their securities (at negotiated haircuts) for which cash investors’ cash.
Every night, the cash sleeps in the (notional) arms of the securities dealers, while the securities (and a promise to buy them back in the morning) sleep in the (notional) arms of the cash investors.
Every morning, the cash wakes up and springs from the dealers’ beds back into the waiting arms of the cash investors, and vice versa etc.
Which means that the dealers need to borrow cash to be able to give it back to the investors. Where do they get the money?
Well, from JPMorgan or BoNY.
Where do JPM and BoNY get the money?
Well, from deposits.
Whose deposits?
Well, the deposits of the cash investors.
let's simmer down folks... 
