[alasdair_]

The standard answer is that speculators provide liquidity.

People that want to buy or sell a commodity typically only want to do a certain number of transactions. What happens if they can’t find someone to take the other side of a trade? It would be annoying to have to wait a week until someone else is interested. Moreover, they may want a different quantity than you do, or they may want a different delivery date.

There is also price discovery as you allude to. A farmer selling pork doesn’t necessarily want to have to track the details of the Chinese economy and the weather patterns in Europe just to get a fair price for their goods. Speculators help with that.