It is essential for teachers to get continuous feedback from their students.  This helps them to plan lessons that meet the individual learning needs of all their students.  Even more importantly is the impact that self assessment and metacognition (thinking about your thinking) has on student achievement.  It is important that students are guided in using these strategies effectively.  They would need to understand that it is good for their learning to ask questions about those things they learnt that might be unclear to them.