In classic economics the benefits of competition are clear enough, but in the monopsony that is the military marketplace the government must pay all the costs for sustaining competition.
In addition, in areas where temporary monopsony power arose and generated rents for employers it would provide firms with the incentive to locate there and bid up the below market wages until they reached the competitive level.
The first is when we have monopoly capitalism (what we would now call a monopsony, a word unknown in his time, a single purchaser of labour) and the second is when there is a reserve army of the unemployed.