There are four main reasons:
What and When to Cite?
You should always cite other people's words, ideas and other intellectual property that you use in your papers or that influence your ideas. This includes but isn't limited to books, journal articles, web pages, reports, data, statistics, speeches, lectures, personal interviews, etc. You should cite whenever you:
Academic integrity, student cheating and plagiarism are concerns of the utmost importance to university faculty, administrators, writing center and tutoring staff, librarians and academic advisors. The short, straightforward definitions of academic integrity and plagarism are meant to assist persons interested in understanding more about these issues.
When using or quoting word for word the words of another person it must be acknowledged. Summarizing or paraphrasing the words or ideas of another without giving that person credit is also plagiarism.