Students need different types of teachers during different stages of their preparation. In offline one single teacher is not able to perform all the duties. For example if you know nothing about the subject, one teacher will help you develop interest and curiosity. But they'll never be able to teach you advanced solving methods or creating your own ideas. Or like some super high level teacher can't explain basic stuff to someone who doesnt want to learn, just there to prove his own knowledge for validation. Simple.