Academic honesty is a very important part of being a scholar. It is the idea that the work you turn in is yours, that it was not copied from someone else's work or done by another person.
When we use information or ideas from another person or source, we must give them credit, or cite. If we fail to cite sources, that is called plagiarism and it is a form of cheating. Plagiarism can get someone into academic trouble.
Luckily, it is easy to cite sources!
Your first answer is probably "to avoid plagiarism." That is one very important reason to credit the sources from which you found data or quotations used in your paper, project, or presentation.
However, it's not the only reason or even the most important. When you cite sources, you are actually participating in an ongoing conversation between scholars. You are indicating to your reader that your work is in response to the work of others, and giving them a path to retrace that conversation. You show the evidence of your thinking, analysis, and synthesis of information when you can point to the sources that helped you make your point. This is how scientific discoveries are made, and how social change occurs. It is very important!
Are there other reasons to cite sources? Add your response in the comments!