[DonaldPShimoda]

Not necessarily. Companies that facilitate trades (eg, E-Trade) don't fall in this category, because their business model is about providing a service to regular people.

Jane Street is a proprietary trading firm. They have no external customers to whom they would provide goods or services. Their primary purpose is to invest the company's own money.

[dqpb]

I think an economist would say market makers reduce the volatility of a market, which does benefit regular people.