That still does not satisfactorily answer the question though, why don't they just sign a purchase order with N month lead time? That still gives a well defined price ahead of time.

But that's literally what a "future" is.

The people who need the constant price do that, but creating this contract generates an investment at the same time. The person promising the price is taking a premium to take the risk, they can the sell that risk in a free market.