So then, you have to offer additional services. HRT has the SDP that they provide, and of course charge fees for. But then the question is why would anyone do this, versus just going through any other financial institution.

The answer is basically all up on their website

"As a liquidity provider, HRT develops automated trading algorithms designed to provide the best prices to our clients".

The question that should be asked is as a user, why would I want to sign up with HRT or any similar financial company? The answer for HRT is because you want to have access to more complicated financial derivatives - you don't need to sign up and pay fees to buy basic stock.

So they promise that their algorithms give the user the best price, which is a legal way of saying that you will pay less for a certain asset and make money on it, and you can't say that because you can't guarantee this.

And its well known that nobody ever gets rich of an algorithm in finance unless you are well established large firm intricately tied with the government that can move so much money as to influence trading.