Many students believe that practicing speaking with other learners is not useful. Obviously if there was only one teacher and one student in the class, then everything that the student says can be monitored and checked for errors. They will get more individual help. But this kind of class is expensive and more tiring both for the student and the teacher. So often this isn't possible and it is better for the teacher and students to have a class.
During the time that the teacher is talking, nobody else can talk. If the teacher asks one student a question, then they can talk, but the others are still silent. If the class only involves the teacher asking students to speak one by one, then there won't be enough time for each student to practice making their own sentences. There is still a lot of value in doing this to begin the activity, so that everyone understands how to do it. Especially if it is a role play or the rules of the activity are complicated. But all students need time to talk and practice using the target language in class, so having pair work in class is very important. Here are the benefits:
Fluency - You can develop greater fluency if you can practice the target language many times. If you are in a group of two with a partner you can both speak 50% of the time. When it is just the teacher talking then the students speak 0% of the time.
Become a Judge - Once you understand the target language, you may be able to pick up on mistakes by your partner. They say that the only way to truly know something is to teach it to someone else. If you can find the mistakes in others language usage, then this shows that you are well on the way to mastering it yourself. You may be forced to explain to your partner why, and if you disagree then ask the teacher. Usually these cases will turn out to be issues that other students might have as well, and as a teacher I can say that most of the time I have been very thankful for these small disagreements because we can look in detail at the case.
Freedom from Pressure- When you are in a group of two you have a little bit more space to try different kinds of sentences, you can also repeat very easily if you make a mistake. When you are speaking and the entire class is listening there is a lot of pressure. Some students even feel a lot of pressure when speaking one on one with a teacher.
Monitoring - during the activity the teacher will go around to each of the students' groups to hear a little bit and check that it is going well. You can easily ask a question or ask the teacher to listen to your sentence. Even if they are sitting at the front of the class basically just taking a rest, you can still put up your hand. This is the best way to get more individual attention from the teacher on the parts of the curriculum that are most difficult for you.