For one thing, they never have to take possession of the commodity, so they don't incur any costs of physical ownership.
The trade process allows the liquidity in the market. When I was a big user of natural gas, I had big tanks and a pump station installed and I had corporate purchasing guarantee those tanks would be filled at a certain price so I could run a casting operation with known energy costs. Maybe spot price was better, I don’t know. But I knew my cost and could plan accordingly. Without traders/speculators (liquidity), unexpected huge price changes would have screwed my end product pricing and shut me down.
For every trade on any stock, commodity, bond etc, there is a winner and a loser at the next tic. There’s always a seller for every buyer and increments are relatively small. Without traders and the at-risk dollars of those elite tennis club armchair players, price/cost stresses would gap to potentially disastrous situations.
Am I missing something here? Someone tell me why free market speculation is wrong. The only types of manipulation that are bad that I’m aware of is cornering a market like the Hunt brothers tried in silver some years ago(and they got screwed) and painting the ticker so as to create a trend - and that’s already illegal and the SEC will get you for doing it.