Hong Kong, despite being officially part of China, can be confusing from a visa standpoint. From a travel perspective, Hong Kong is still separate from mainland China. You'll go through customs and immigration and if you don't have a return visa for China, you'll be ending your trip there. (Don't panic, you can apply for another visa to China from Hong Kong, but you'll have to hang around Hong Kong for a few days.)
For Americans, getting a re-entry visa makes a lot of sense if you're not sure if your itinerary will include Hong Kong, but it might. The re-entry visa is the same price as a single-entry (and a multiple entry) so it might be a good choice for you.