Maybe I can answer.
One thing it does is it allows airports to work a little bit more flexibly with CBP in terms of the allocation of resources. In some airports, particularly if the hours of service are not really right for a flight—for instance, if a flight comes outside of hours of service, which may be more of a factor for some of the smaller airports—with pre-clearance they at least have the ability to supplement the services. They can control the amount of resources provided for better connecting times and competitiveness.
So part of it is the greater flexibility, really, over the resources that are available to CBP.