It's important to cite your sources, for a variety of reasons:
To give credit to the author(s) of your sources.
To allow your readers to locate your sources.
To show that you thoroughly researched your topic and can support your claims.
So you don't get in trouble for plagiarizing.
"If it is not your original words or ideas, give credit to the person whose words or ideas you are using," advises the UA's Dean of Students Office. Plagiarism violates the UA Code of Academic Integrity. You don't want to face these sanctions.