When you hit "buy", your trade is matched on an exchange in microseconds — but the shares and the cash do not actually change hands until T+1 (one business day later; it was T+2 in 2021). For that gap, somebody must guarantee that both sides perform. That somebody is the central counterparty (CCP) — in US equities, the National Securities Clearing Corporation (NSCC), a subsidiary of the DTCC.