The reasoning is that bidders will offer keener prices if they perceive reasonable odds of success. Also if they understand your business better they may see your custom as less risky, which can lead to lower mark-ups.
The balance of risk plays an important part in getting good prices. The less uncertain a potential supplier is about your expected consumption volume and daily or seasonal profile of demand, the less risk premium will be added to his quotation. This is most true under the New Electricity Trading Arrangements (NETA) where your supplier must place wholesale contracts which balance the aggregate of all his customers' expected demands, half hour by half hour. Customers who upset the balance incur high costs to the supplier, either to pay for the additional generation which had to be brought into service or compensating generators to forego income which they had contracted for.