The answer is: The Greeks held the athletic games to honor Zeus.
The Olympics were intended to be a solemn ceremony where ancient Greek belief gave honor to one of the major Gods: Zeus. While these games were held, all differences from the city-states were conciliated. All political hostilities were paused. Unity was promoted by reinforcing cultural religious practices from the Hellenic world. The Olympic stadium was attributed to Heracles building the place as a temple for worship to Zeus. The ruins we see today are a historical record for a major sanctuary from the time.
