Beginners avoid silence. That’s because they sense that it's in momentary silences that an audience can most easily see through them. So they mask their vulnerability with 'um'. So, it's understandable but wrong and we all know it.
The goal, of course, is the opposite—to be a presenter in such relaxed control that you can hold deliberate silence in appropriate places. The longer you can hold silence, the more status the audience will award you.