Cash trades are bilateral: a specific cargo, a specific elevator, a specific delivery window, with credit terms and freight attached. Prices are usually quoted as a basis to a futures month rather than as a flat number, which is why futures matter even to people who never trade them.
Price discovery flows both ways. Futures give the cash market a reference; cash tightness drags futures into backwardation.
Example: a physical crude cargo prices as "Dated Brent plus $1.20 CIF Rotterdam". The flat price only becomes known when the futures reference is fixed during the pricing window.
Related: basis, convergence, exchange-for-physical, forward-contract