Taken from MICDS McCullough Library:
To back up your arguments. You want your thesis/ideas to be trusted. By showing evidence you are demonstrating that you aren't just making up the facts. The better your sources, the better your argument looks. If you needed immigration statistics, getting those numbers from the US census is far more credible than reporting the information that might be found on a Buzzle article. Put it another way, would you want to get your health information from a doctor that only searched Wikipedia?
To demonstrate your research. In a high school history class, you are probably not the first person to do research on a particular topic. There are many historians/experts that have done the work before. You will probably see their names mentioned several times in your research. By quoting and citing them you are demonstrating that you really know your topic and can prove it. Your teachers know who these people are! Teachers can also look at a bibliography and judging by the sources you used they can guess how good of a paper/project it is going to be!
To leave a research trail. When you provide a bibliography/citations you are leaving a trail for others who read your work to go and find additional information. In college, you will probably be searching bibliographies and citations for sources and will appreciate them greatly!