Attending a class where discussion is always dominated by the same handful of confident students can be annoying. It’s not great for learning, either: Participation is a form of practice, and hearing from a broad selection of classmates enhances everyone’s education.
For those reasons, many professors use “cold calling,” picking out a student who did not volunteer to contribute. Or they might use the related practice of “random calling,” found mostly in large STEM classes, in which instructors select a student or group to hear from using a random-number generator, names from a hat, or a similar tool.