1. To avoid plagiarism.
2. To add credibility to your work by demonstrating that you have done proper research and consulted other sources before you write on a topic.
3. To help the readers, including your professors, to track down the sources you have used.
4. It is a standard practice for scholars and students engaged in research and writing. It is the ethical thing to do. You are being a responsible scholar when you cite sources and acknolwdge other scholars for their ideas.
Look to the right and under the Tools, click on Cite.