The clearinghouse collateral requirements in part protect the clearinghouse from things that can go wrong at the brokerage, like if Robinhood had a vulnerability that let people place huge orders without paying for them, and they were, like, put out of business overnight.
I don’t think it has anything to do with margin vs non-margin accounts for instance. It’s just a formula where you split up the outstanding shares by the VaR as I’d get a capital requirement.
B) your shares trading immediately is a fiction. It takes days for those trades to settle. And any subsequent trades you make with those funds are all subject to credit risk. The central clearing house collateral rules are about risk management around that multi day float.
I think but am not an expert on this that the DTCC times explicitly require the capital to come from the brokerage not the clients. I don’t know why but can guess that it’s because it’s the brokerages taking on the risk not the individuals.