There are two reasons for this. One is "legal," or technical, and the other is ethical or moral.
The first one reason is that you would not want to get in trouble. From a purely pragmatic point of view, you should identify the sources that you have used so that you do not get punished for plagiarism. Of course, this is simply about avoiding punishment; it is not a positive reason why you are ethically obligated to identify your sources.
The ethical reason is that you should be honest about what is your work and your ideas. If you simply take other people's work and ideas and pass them off as your own, you are essentially stealing. You are also lying because you are saying that something was your own idea when it really was not. Both stealing and lying are unethical and therefore you should cite your sources.
Overall, then, you should cite sources both to avoid punishment and to be sure that you are acting in an ethical manner.