Hong Kong is located at the mouth of the Pearl River Delta--the largest manufacturing area in the world. A substantial amount of the output from Dongguan, Shenzhen, Guangzhou, Zhuhai, etc. comes to Hong Kong and then is shipped on to its ultimate destination.  

Hong Kong is typically the top container port by volume in the world or a close second behind Singapore depending upon the year.  Hong Kong International Airport is also one of the busiest airports in the world for air freight.  Nearly all of the exports out of Hong Kong originated somewhere else. While it used to manufacture, nearly all of Hong Kong's manufacturing has moved to China and Vietnam, amongst other lower cost locations.  Lastly, Hong Kong is not a huge city (less than 8 million people). So, most of the container and air freight volume coming into Hong Kong is destined for places outside of Hong Kong.

With a large well developed container port and airport and and as a free and open port without customs formalities that cause delays or duties and taxes adding expenses (with some exceptions), the movement of goods from places near Hong Kong (such as Southern China) into Hong Kong for ultimate export to the U.S. or Europe is easy and makes sense from a transportation perspective.