Suppose we're giving out 100 promotions, and have 1000 promo nominations. You review the 1000 noms and find that there are 500 that pass the quality bar because your company is full of awesome people. So now you have to choose which 100 of the 500 to actually promote. You can do this in a number of ways, ranging from uniformly at random to doing a total stack rank and selecting the top 100. Or you can choose to try to correct systemic under-representation in management, which helps the company on a number of axes (including better recruitment of smart people from under-represented groups, providing a statistical bulwark against lawsuits, etc...).
(edit: It's worth keeping in mind that trying to do a full stack rank of your 1000 applications is going to be noisy as hell and an incredible amount of work, as there will be multiple (individually noisy) promo committees working on sub-sets of nominations. So it pays off to have a clear set of standards for promotion to a given level, even if you don't automatically promote everyone who meets that standard.)