Describe the benefits of the open door policy on china

This refers to trade with China and was coined in the very late 1890s - 1900 to describe the kind of policy that the United States wanted to promote. Technically, they wanted protection for every nation that traded in China guaranteeing equal privileges among all. A sort of "play fair" rule for everyone that tried to also protect the sovereign rights of the Chinese to prevent their country from being carved up.

 It was supposed to level the playing field, though most nations preferred to ignore it and play nice only when they needed to. In those years, China was ruthlessly exploited by Western powers and did little to curb some of their activities. It pretty much came to an end in the early 1930s when the Japanese militarily expanded their hold over China. 

In more modern times, following the late 1970s under Deng Xiaoping, it has come to refer to China's increasing openness to foreign trade and business. Before that, it was a largely isolated country that wanted little to do with the West. 
The benefits today are that China is now the #2 economy in the world (having beaten Japan some time ago) because of its growth through manufacturing and trade, a rapid industrialization of the country to more modern standards, and rising quality of life within the major cities (access to goods from around the world). For many large corporations, China is seen as a huge market as well as a giant source of cheap but reliable labor. 

I hope that this is the answer that you were looking for and it has helped you.