Curriculum ensures teachers plan the whole term or unit of lessons and are proactive not reactive in their teaching.
Proactive teaching happens when the teacher directs and or guides the student to enable the student to achieve their personal goals. Proactive teaching means the teacher has a planned program to suit the students needs and is constantly checking and evaluating that all aspects the “curriculum” are being covered.
Reactive teaching happens when teaching is always in response to what the student does in the lesson. Let me clarify, all teachers need to respond and adapt to what the student brings to and does in the lesson, but if this is the only kind of teaching going on and there is no longer term plan, no list of technical skills, no planned repertoire, no development of performance skills then the student may not achieve their goals and may wander aimlessly in their learning.
Curriculum ensures that we teachers plan what we teach and check that we are teaching all the aspects of music learning suited to the level of study.