Unlike simple computational problems in math or fact-recall questions in social studies, open-ended questions allow students to find more than one correct answer. Because these questions often end with "Explain your answer" or something similar, students must be able to express their reasoning and prove that they have a deep understanding of the concept. Teachers can use these responses to understand how their students think and where their errors are if they do not find a correct answer.
Open-ended word problems in math and analysis questions in social studies or literature can require students to integrate concepts in their answers. For example, the question "Jim's toy has four sides and at least one 90-degree angle. What is the probability that it is square? Explain your answer," requires a student to apply his knowledge of geometry and probability and then to articulate his reasoning. A literature question may ask students about the social currents of the time in which a book was written. This teaches students to take their reading out of the vacuum of the classroom and consider it in a wider context.
When questions elicit multipart answers, they allow teachers to give partial credit for reasoning or understanding, even if a student does not answer the question correctly. This provides a more accurate system of assessment, particularly in mathematics classes, where a student can lose all points on strictly computational questions if he makes one careless error. If a student demonstrates solid reasoning on a question despite giving a wrong answer, the teacher can give him partial credit so his score reflects his understanding of the subject.
School is not the only place where open-ended questions prove effective. Teenagers and adults are likely to encounter them in college applications, at job interviews and on dates. An interviewer can learn more about an applicant by asking "How does your current job reflect your strengths?" than by asking "What are your current job responsibilities?," and high school seniors are likely to find questions like "Explain why you would be a good fit for our university" on their college applications. Open-ended questions like these allow people to keep conversations rolling and to learn more about the person answering them.