Great teachers are those who've discovered new aspects of reality. Who then presented this information to the rest of Humanity in a comprehensible form.
It is those who've walked the braves of the void before us, that taught all ancestors in all parts of the world some Way to live.
Without teachers to guide us on our paths, the everyday reality we live would be very difficult to understand and we would possibly be doomed to repeat our mistakes.
The basis of Samsara.
Teachers act as powerful instruments of inspiration for Individuals who understand The Way.