Formative vs. Summative Assessment
Most teachers hear the word "assessment" and think instantly of tests or essays. While these are undoubtedly assessments, they are not the only kind. Tests, quizzes, and other graded instruments are often used primarily as "summative" assessments, which means they exist only to evaluate, to give a grade.
Less heralded but equally important are "formative" assessments, which are used to elicit feedback and to facilitate learning. Ungraded Classroom Assessment Techniques (CATs) are examples of formative assessment, so named because it helps the teacher "form" an opinion of the students' level of understanding (equally important: the students themselves can "form" an opinion about whether they really know the material).
Formative assessment is obviously useful for judging whether it's safe to move to new topics (or the opposite reaction: learning that the material must be taught again, hopefully via a different approach or technique). The magic of formative assessment is that it often forms the very moment of learning. Only when asked to perform do learners sometimes truly comprehend the material, and these performances often do much to ensure that the knowledge is coded in the brain for long-term storage, and thus recall.