Learners differ, and a method that works for one group fails for another. The single most powerful learner characteristic is prior knowledge: new learning attaches to what the learner already knows, so a teacher must find the starting point before teaching.
Worked example: two classes must learn fractions. One class already handles division well; the other does not. The same lesson plan fails the second class, because its learners lack the base the new idea attaches to. The teacher who checks prior knowledge first can bridge the gap instead of discovering it in the examination.