One of the most important ideas in economics, and especially in finance, is the Law of One Price.
If wheat is trading at five dollars per bushel in New York, then it shouldn’t be trading at ten per bushel in Chicago. That’s a huge opportunity to buy low and bid up the price in NY, and sell high and push down the windy city price, until the two prices match. Arbitrage. In equilibrium, the prices should be essentially the same. It’s a standard assumption of neoclassical (equilibrium-based) finance that we look at the world after equilibrium is established and the arbitrage opportunities have already been exhausted. There are no such opportunities left.
Repo transactions between private banks are collateralized lending (a little different from “repo” agreements with the central bank which can create its own cash).
It’s like a pawn shop. You want a loan, so you bring your granddad’s gold pocket watch to the shop for some cash. You must then repay the loan, with interest, or the pawn shop gets to keep the watch. In this case, the “gold watch” is government bonds. This kind of lending is safe because the lender is sitting on an asset that it gets to keep if the borrower can’t repay.
No arbitrage opportunities left: The price of wheat in NY and Chi should be near identical; repo interest rates should be roughly the same as any other lending opportunities that represent extremely little risk. Because if repo pays better than those other opportunities, then why would banks not move out of the low-interest safe assets, and into the higher-interest safe assets? It’s free money for the taking.
The Fed pays banks to hold cash and not do anything with it. (This is puzzling behavior, but they do it anyway.) This is a perfectly safe return for banks. The repo market is also extremely safe, not perfectly so, but close enough that the rate on repo should be approximately the same as the rate for excess reserves, if cash is allowed to move freely between the two places. There’s no reason to earn only the safe excess return rate from the Fed, if a bank can use that same cash and easily earn more than that with an almost equally safe asset. Repo rates should not be much higher than interest on excess reserves.
Yet we’ve also seen interbank repo rates periodically spike in the last year. As if there’s no cash left. As if all that cash in excess reserves is no longer free to move.
Because, of course, it is not. Banks were requested to use their excess reserve cash for regulatory requirements, which meant it couldn’t be arbitraged with the repo markets, and that is exactly why repo rates have been periodically spiking, and why the Fed started intervening so massively in the repo markets themselves in the last year. “Excess reserves” are no longer excess. I mean, sure, they’re still in excess of deposit regulations (hence the name), but they’re no longer in excess of capital requirements.
The Fed is intervening in repo because that’s where they weren’t getting any traction using their other policy tools.