Academic writing often requires sources. You will use sources to support your main idea. Using good sources makes your writing more credible. It is an appeal to ethos. By citing others you show that others share or support your idea too and, hopefully, that those other people are experts in some way about the topic. You also show that you have researched the topic and have become an expert on the topic yourself. 

You need to be very careful when you use sources so that you do not plagiarize someone else's work. Quoting, summarizing, and paraphrasing are the most common ways to use sources.