The language teacher makes the students learn new words and practice their pronunciation. This is important because they can sound totally strange to your peers and the person in front of them. As such, the teacher will train students so that when they speak the language, they sound natural.
Once the student has become familiar with the language learning style and with the pronunciation of words, they will be able to speak normally. However, if the student chooses to study the language and feels the need to become fluent, then he can do so by working with a teacher. The teacher will work with the student so that he learns and understands the language he wishes to learn. Once the student is able to learn and understand the new language, then he can learn to use it effectively.
A good learning program will have a teacher who will help students in their particular area of interest.