They add liquidity to the market, so when you (or someone indirectly on your behalf) buy or sell stocks, you (they) get a better price through narrower bid-ask

They add liquidity to the market, so when you (or someone indirectly on your behalf) buy or sell stocks, you (they) get a better price through narrower bid-ask spreads. They also sometimes generate comedy[0]. [0]: https://www.henricodolfing.com/2019/06/project-failure-case-study-knight-capital.html https://www.henricodolfing.com/2019/06/project-failure-case-...