Open ended responses allow students to think through their answers, and more importantly, it teaches them to think before they begin to write their response. Another, greater, value is allowing students the opportunity to actually look at the source, making judgments on their own first, before reading possible answers in a multiple choice set. This turns on the analysis process in our brains, and helps us to recall skills and content as we step through the stages of thought.
However, the greatest benefit to prepping with primary source analysis materials is the fact that students are practicing so many skills in one. Addressing standards can be a challenge in the Social Studies classroom, but in using primary sources students can see images, read quotes, evaluate charts and graphs, use maps, and read passages to help them better understand the cause and effect that is so relevant in any social studies course.
Take a look at my Primary Source Materials for great reviews of U.S. and World History, Geography, Government, and more!
For a very comprehensive review for U.S. History, take a look at this Amazing U.S. History Primary Source Analysis Bundle!