Here is the list of institutions who used RRP in January, it’ll be much the same today.
https://www.financialresearch.gov/money-market-funds/us-mmfs-repos-with-the-federal-reserve/
Basically, anyone with a large business in US govt money market funds who has more cash deposits than it can invest in the sort of things that money market funds buy, and other major US banks who have more cash than their balance sheet has room for and that they have loans to make, uses the RRP sometimes. It is a place to park cash overnight, and now that it pays 0.05% interest (annualized) more cash is getting parked there.
Normally, money market funds and banks will have other places to park money, like T-Bills. But the US govt isn’t issuing as many T-bills as the market desires, so the interest rate on T-Bills is zero or occasionally even slightly negative. That is a problem for money market funds, who are losing money - they pay 0.01% interest, are waiving their fees, and earning zero, not good. That is also a problem for banks, who operate under regulations requiring liabilities (a customer deposit is a liability) to be supported by assets (like loans) or capital, but right now have more deposits than they want.
So, the Fed steps up to its role (one of its roles) as the central bank and takes that excess cash as, essentially, overnight deposits.
As I said before, this is one symptom of too much cash sloshing around without enough productive uses for it. As symptoms go, it seems pretty benign to me. I have yet to hear a concise explanation of how Fidelity having $50 billion of overnight deposits at the Fed is going to kneecap the economy. There are other symptoms of too much cash that are not benign, this particular one gets more attention than it deserves, in my view.