The purpose of Hay's open door policy was to ensure free and open trade for the United States and other countries with China. The policy was worded so that it not only protected U.S. rights there, but also China's rights as well.
The policy was meant to ensure that none of the countries who held interest in China would reduce the levy fees at any given port for their own countries, thereby lowering the amount of taxes that China would collect. The tariff was also to remain untouched in any spheres of interest or ports. The idea was to keep things fair and equal for all countries, whether they were leasing land in China or simply trading with the country.