Answer: to assure that the United States had access to trade with China.
The Open Door policy was issued by the United States in 1899-1900 as a series of dispatches from the US Secretary of State to other nations that had trading interests in China -- Great Britain, Germany, France, Italy, Japan, and Russia. The policy reasserted earlier agreements that all countries should have equal access to ports in China, with no favored "spheres of influence" for one nation or another. The United States was seeking to maintain an equal footing with other nations in the access to trade in China.