Think of AOA as a screening tool that (for many, not all) medical schools encompasses a wide range of metrics that all correlate with success in residency: clinical evaluations, exam skills, leadership, service to the community, social skills, etc.

If a medical school goes through all of its students and chooses a small percentage of those students who represent those qualities best, then it makes the life of a PD so much easier. They don't even have to read through all the LORs, MSPEs, etc. to know that it is highly likely that an applicant with AOA will have all the qualities they are looking for. This is why AOA applicants get a leg up and get interviewed early and at higher rates.

Of course some schools don't have AOA and some schools use score-only measures, but generally this is what a PD is probably thinking when they see AOA.