So who is on the buy-side? Exclusively supermarkets/distributors, while exclusively farmers sell? I suppose that could work, but I assume it would quickly regress into tight relationships like we have (probably regionally variable) for smaller market's, like most vegetables (vs grain) where as I understand it it's largely a direct relationship with the buyer - you probably still sell a future contract, but it's not via a central market and it is 'farm x will deliver to buyer y', i.e. a pre-order if you will, not really a commodity.
And as others say, price discovery, liquidity. What harm does completely open (no obligation) do? And maybe you eat a lot of potatoes and want to lock in the price today. (Or more seriously maybe you're a big baker, but not big enough to be buying direct from farm, your miller is. So grain price affects you, but ypu can't directly control/choose when to take it. Secondary grain futures allow you to hedge risk of it moving against you. In turn this means lower prices or lower risk of shock price increase to your consumers.)