Brokerages pass on trades to market makers. They don't promise a price, they just tell you which price is available through market makers, and when you decide to go ahead with a trade, they go on the market, and you hope that the price market makers were advertising earlier is still what they're offering when you come through. Brokerages make a profit by passing on your order and taking a percentage of your total sale, not by making a profit on the spread.

I just find that whenever anyone wants to justify a total waste of resources like some people using microwaves to shave a micro off to nickle and dime the world it always comes down to some guff about liquidity to defend it.

The arms race where you never get to zero.

Providing liquidity is a service, so institutions expect to be paid for that service