Some students might do all the talking because they are cleverer and I find it difficult to think quickly enough to comment. In both cases, the groups are relatively small compared to the number of students attending lectures. It is a chance to check and clarify understanding and make sure that the main issues are clear. The tutor may still do most of the talking but students are expected to comment, ask questions and check understanding.
Tutors are present to comment and help.
Seminars are an important way of learning - by discussing and questioning issues, students can clarify their own ideas and also learn from each other. The ability to articulate ideas is also an important life skill which will be required outside the academic world in the world of work, for interviews, consulting experts, getting and understanding advice and giving work presentations etc. Seminars give practice in these general skills and help students to develop confidence. They also underline the democratic nature of learning-everyone has the right to express their point of view and to comment. We learn about learning.