What is the purpose of ICAO Aviation Security Audit?

to determine the status of security standards within contracting states

ICAO: the mother of aviation growth and safety.

ICAO (International Civil Aviation Organization) simply set the rules of the basic standard procedures airlines need to follow. these rules are followed by different states, and in certain states they are added upon, then legally enforced in order to unify the standard procedures followed globally. this promotes continuous safety and growth of the aviation industry. 

If you are referring to the ICAO Universal Security Audit Program (USAP) which takes place in each state that is part of ICAO. the audits should "be focused on a State's capability to provide appropriate national oversight if its aviation security activities."