The most common reason is to avoid plagiarism. Plagiarism is taking someone else’s ideas and presenting them as your own. Plagiarism needs to be avoided for two main reasons:
As an act of respect for the other person. Original ideas are difficult to come by, and claiming someone else’s as our own is disrespectful to that person due to the work, research and effort that it took him to get to his conclusion.
It is also a way to track erroneous information. If someone makes an argument, and that argument is wrong, we can go back to the original source and prevent misinformation from spreading.
Another reason for citing is that it helps us look at the progression of information. This can be useful if we are interesting in seeing how ideas spread or how ways of thinking change through time.
