A large online class can get too boring at times, and students always need refreshers every now and then. The questionnaire and quiz sessions can provide that much needed break in these large classroom environments. Organizing a quiz/questionnaire on a weekly basis can help the students in developing interest towards the topic and getting regular clarifications on the current topic.
During these sessions, you can also find out the less-interactive students, and find out their problems. Sometimes, many of the students may be pretty good at writing and answering questions in written form, though they may not participate actively in such interactive sessions.
The main aim of organizing these sessions must not be to find out the brightest of the students in the class, but rather to figure out the ones that need more attention.