Most retail brokers are too small to be a bank's direct prime brokerage client. A prime of prime aggregates their flow, posts collateral upstream, and passes through pricing with a markup, effectively renting out access.
The chain adds a layer of counterparty-risk: client, broker, PoP, prime broker, bank. Each link has its own capital and regulation, which is why account holders should know whose balance sheet actually holds their money. See segregated-funds.
Example: a retail broker routes $2,000,000,000 of monthly volume through a PoP, paying roughly $3 per million in markup, about $6,000 a month, in return for credit it could not obtain directly.
Related: prime-broker, counterparty-risk, liquidity-aggregation, segregated-funds