Many of these firms are market makers. That’s a role that has existed in markets well into the 19th century. Market makers are contractually obliged to offer a price for any security on their books. That means if you want to sell, you always can no matter what is happening in the market (unless it’s suspended). Nowadays all market makers (that I know of) trade algorithmically and given that volumes and speeds at which modern markets operate, it’s pretty much the only way they could without having to charge considerably higher market maker fees, increasing costs for everybody.

Furthermore, if I want to sell a security and one of these companies puts in the best bid, then I just made more money than I otherwise would have if they weren’t there. Likewise if I was buying, they might offer me a lower price. Isn’t that a good thing? They are happy and the counterparty is happy.

Then there’s arbitrage. If I am trading on an exchange, but there’s a better price on another axchange one of these firms trades on, they might offer me a better price than I otherwise get. Effectively I d get a price on the other market, without having the costs of trading on it. Again, that’s better for me, where ‘me’ might be your savings account or pension fund manager, or a manufacturer or producer on a commodities market. So these firms reduce costs for other participants on the market.