All of those firefighters, police officers, and teachers can't get their monthly check unless the pension funds converts part of their investment into cash, and the amount traded gets so large you need well-funded financial participants who will pay the pension fund a large lump of cash and assume the risk of unwinding the position.
The same thing goes for portfolio rebalances. Some smart and prudent retirement fund has run their risk model and decided that their sector exposures are off by like a few percent. They've got 2% too much financials, 2% too little tech, they want to move 1% from consumer staples to consumer discretionary, etc. We're talking about billions of dollars being moved right here! They want to use the money from the assets they sold to purchase the assets they bought, but they want the transaction to happen all at once... not in two parts. Again, this is where the quant funds and market makers of the world come in.
It's easy to think that finance is all fat cats, hedge funds and multibillionaires, but anybody with a retirement plan, a pension, ETF holdings, or mutual fund holdings, benefits from the work these quant funds do.
