Properly citing the books, articles, and other resources you draw from, enables others to trace back to the evidence you've used to support your arguments and claims.
Citation errors are not only distracting, but can also potentially mislead your readers by making the false impression that you're appropriating others' ideas or work as your own. As a researcher, you'll want to track down the citations you come accross in a works cited or reference list. Incomplete or incoherent citations will make it frustrating to locate what you need, hence the use of standardized citation sytles.
There are many free online resources that will give you an overview of APA. These are a few favorites: