The idea behind the spiral curriculum is that topics once covered in class have to be revisited to ensure everyone fully understands an idea or concept. One of the main reasons children and young people fall behind in school is because they have insufficient time or stimulus to fully understand things. A teacher who only teaches something once may have taken some young people with them, but there will almost certainly be a group left behind.
Good teaching is not monotonously going through the syllabus and textbook. Good teaching means revisiting topics to deepen understanding and ensure no student is left behind; the spiral curriculum concept is a metaphor to remind us about the importance of the revisiting process. After all, if you are explaining something to your own or a relative’s children, you do not necessarily expect them to understand immediately. You might have to explain things in two or three different ways before they fully understand. The same process applies in the classroom. School knowledge needs to be organised in ways that allow students to think through ideas from a variety of perspectives.