Why is it important to cite other people's work in your research?

You must give credit to the source of your information on a written document or an oral presentation, to do otherwise would be to commit fraud. The offense is titled plagirism. It means you have taken anothers property (their knowledge) and presented it as your own rather than giving them credit for the information. Simply re-phrasing an idea may or may not be enough to rise above the offense but its best to stay on the up and up.