Why do I need to evaluate the resources I use?
Not all information published in books or on the internet is credible or approriate for your needs.
Your project is only is good as the information you base it on! For this reason, it is important to make sure the sources you use are credible or at the right level for what your are doing.
Why do I need to cite the sources I use?
By saying which sources your information is from, you give credit to the people who did the original research. Not giving credit is plagiarism.
Also, by saying where you found your information, you give people who read your work the ability to look at that information and read more about the topic.