It gets you a real price: no supplier can see what a rival quoted, so bids are priced off actual cost rather than tactics.
For a buyer, that removes the padding. In ordinary haggling a seller positions the number by guessing how many people you are talking to and where the competition sits. On MarbleMap that information does not exist: a supplier sees neither rival offers, nor their prices, nor even how many offers have arrived. All they have is their own cost and the wish to win the job. The result is a serious number in the first round.
For a supplier, the gain is the absence of a race to the bottom. With no rival figure on screen, you never burn margin just to slip under it. The same shield covers buyers, who cannot see one another either. This secrecy is not left to good faith — it is enforced by design and leak-tested.
Only the buyer sees and compares the offers as a whole. The system flags the most advantageous one; the final call always rests with a person. Because MarbleMap takes no share of any sale and earns only from subscriptions, it has no incentive to steer the outcome — that is the only way sealed-bid fairness survives.
Also asked as: What is the benefit of sealed bidding? · Why do you keep offers hidden? · What does sealed-bid do for me? · Why can't suppliers see each other's prices?