You can buy preferential treatment: If you set a higher price for your buy order, then you are preferred over every lower-priced buy order. If you select a par

You can buy preferential treatment: If you set a higher price for your buy order, then you are preferred over every lower-priced buy order. If you select a participant at random, you are encouraging each trader to place many orders (at the same price level), to increase the probability that one of his orders is chosen randomly. If you use a pro-rata algorithm based on order size, you are encouraging each trader to inflate his order quantity, and cancel very rapidly after all (or nearly all) of his desired quantity is filled.