If you use texts and ideas from others in your own document you must refer to the sources. Otherwise you make yourself guilty of plagiarism.
Who uses literature and sources in a thesis or publication should refer in a proper manner. Readers can thus determine on what your work has been based, if you have used enough sources. And whether those sources are reliable and used properly.
You will find a detailed explanation of the APA reference guidelines with lots of examples at our Digisquare.