First, teaching is the best way to advertise ones own research, and to excite student interest. For example, I have decided to work with my advisor after I took a class with him. Upon completing his class I chose a direction for my research. Second, students are our vehicle to communicate our ideas to the world. In general, it is difficult to convince the real world of new concepts and systems.
Teaching is important because students form the link between the academic and the industrial world.
Third, teaching means explaining, and explaining means learning. Before one teaches one has to order the essential ideas, which leads to a deep understanding of the field. Fourth, active students who ask questions stimulate thinking. It is the critical dialog about a topic which reveals new insights and sheds new light on old questions. Students are absolutely essential in this process.