The Common Core State Standards are requiring students give evidence and explain their answers - spoken and written. Here is a good article to give perspective on the benefits of this process. Students will be assessed on this process on the CCSS assessments - coutesy of ASCD SmartBrief.

Why teachers should ask students to explain their answers
Teachers can help students learn by asking them to explain their work -- rather than memorize and repeat answers -- researchers have found. "We know generating explanations leads to better educational outcomes generally. When children explain events, they learn more than when just getting feedback about the accuracy of their predictions," said Cristine H. Legare, an assistant psychology professor and researcher at the University of Texas at Austin. Education Week (premium article access compliments of EdWeek.org) (6/5)