Here is a link to a site at the Federal Reserve Bank of Richmond which will tell you all you need to know about the repo market.
In my experience repo traders were woefully mistreated (a little hyperbole) but in the caste system which often prevailed in a Wall Street trading room ,they were riding in the back of the metaphorical bus.
However, they conduct the most basic and most important of the firm’s business. They finance the long positions and “reverse in” the shorts. The trading business at the end of the day is about the back and forth movement of money and securities and they make it happen efficiently.
I would say that my own understanding of that piece of the business is inadequate and flawed and if I had the power to run a dealership,I would mandate that every salesman and trader should spend several months learning and understandind and appreciating repo.