Primarily, an instructor is a facilitator for students who are motivated to learn the material offered in a course. The instructor is one among many potential resources a student has to draw on in learning the material: the Internet, the textbook and supplemental readings, other students, discussion groups. The instructor's proper role is to provide one or more paths that students may follow in learning the course material. Depending on the subject and the skills the student is required to master, this laying out process may include choosing a textbook and/or supplementary reading materials, preparing PowerPoint lectures, working problems on the chalk- or white-board, or class discussion. The instructor's primary job is to convey the knowledge and or skills to be learned in the course offered through whatever media are suitable to the subject. Ideally, the instructor also should be available to answer student questions about the material, outside as well as in class. This may include office hours and answering student emails.
A secondary, but related role of the instructor is evaluation of the students' learning of the material and/or skills. Evaluation—whether of homeworks, papers, quizzes, or exams—is primarily to give the students feedback as to their progress and only secondarily (in most cases) to grade their performance for certification purposes. Corrective feedback is one of the resources the instructor provides to the student. Certification of the student's capability with the material is important mainly in that it suggests to the student whether they are ready for future course-work requiring the knowledge and skills taught in your course. (For fields like medicine, there may be additional public safety concerns related to an instructor's certification.)
The last role of the instructor is that of a motivator/inspirer. The best, most natural teachers have the ability to inspire interest in the material they teach: the ability to make the material interesting to the wide audience of their students. Failing that, the instructor should at least make it clear to the students how the material they are learning is likely to be important in their future work or studies. For example, in my opinion, the teaching of mathematics would be better served for engineering students if they were given examples of how the mathematics they are learning will be used in their future coursework.
In summary, an instructor is a study facilitator, an evaluator, and a motivator, in that order of importance. Learning rests with the capacities and willingness of the students to assimilate and apply the knowledge and skills the course aims to teach. The students are the critical agents; the instructor is a resource to those that are willing and ready to learn the material.