Prior to this shit storm it was generally used to fund operations and pay quarterly taxes to the IRS.
Some businesses will have to pay hundreds of millions all at once to the IRS each quarter and they use short term REPO to make that payment.
A lot of its just funding shifts by the financial markets to send the money where its needed
the next biggest bucked is, it allows huge entities to make a quick buck overnight for cash they have on hand instead of making ZERO.
so a huge 100 billion dollar mutual fund can take the 20 billion they have on hand in cash they are not ready to invest in the market and just loan it for a few hours and collect interest.
1 billion dollars loaned for .02% interest is still a bunch of green backs. 200k greenbacks, I think.. quick head math....