What´s the purpose of teaching?

I had the honor to be a three-day-substitute for a German teacher in my former upper secondary school . During those exciting classes, when students were focusing on their tasks and I was sitting on my chair and rolling it around, I let my fabulous mind wander. What is the hidden and highly classified purpose of teacher´s existence? We all know they try to teach students, who are supposed to comprehend all information given. But could it be said that teacher´s goal is to make himself needless? So , they`ll try achieve the point where the students don´t need them? Am I right or right? Or are there any right answers at all?