Ow are Primary Sources used in History?

How are primary sources used in history? what can we learn from primary sources?What should we be aware of? Cite specific examples of how your understanding of American history was changed/improved by sources that we read. What source was most helpful/memorable? Why?
Sources read:
I Came a Stranger; The Story of a Hull House Girl by Hilda Satt Polacheck
World War I Soldiers Diaries:Diaries from the Ste Mihiel and Meuse-Argonne Battles, September 17, 1918-November 11, 1918
Briefs and Supreme Court Opinions in Korematsu v. United States, October Term , 1944.
1968 in America by Charles Kaiser