<i>&quot;So why, if they can always buy at wholesale prices, should they deliberately pay retail prices instead?&quot;</i><p>It is worse. Wholesale prices vary during the day. If you force them to buy at average price at times when wholesale prices are low and sell at average price when they are high. That will cost them money, potentially lots of it (nowadays, wholesale prices can go negative when there is a lot of supply from wind or solar farms)