Certain buyers might want to lock in at a particular price or rather buying at a predetermined spot price. If by going to the open market, generally you don't have the luxury getting such large amount at the same price even if you spread your buying order going to different exchangers. That is because price will start to rise as the market cannot cope with such huge demand at any particular time, and it will push the price upwards. Essentially what it means is that you will be buying at higher price.