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Why? For immediate Q&A! Not only do you get your own questions answered while you still remember, but you also get to hear other classmates' questions (and answers)! Many times I have thought, "That's a good question I neither would have thought of or known the answer to." I might realize that I do not understand the presented concept as well as I thought I did, or my classmate's question/comment might inspire new ideas in me. Gathering many people "in person" means gathering perspectives. Obviously, the smaller and more discussion-based the class is, the more important it is to have it "in person."
For example, I like Prof. Krieger's 7.20 lectures a lot. Human physiology may sound like a "straight memorization" class, but physiology is in fact much more complex. For the aspects of physiology that are truly "straight memorization," Prof. Krieger tells us "I won't go through the names/details/etc of ____, because it's in your book." (We have reading assignments in our textbooks.) In class, Prof. Krieger teaches us how to synthesize and integrate the different things we have learned (or memorized) and presents material in a series of "why" and "what if"-type questions asked to the class.
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Actually, online learning is great - sometimes better - for Q&A.
I don't think being in-person is required for asking questions. I took an online course where the speaker presented on a video/webinar stream and the students had chat on the side during the presentation. The speaker and the class administrator and TAs could be on the chat as well. This way you could ask questions real-time. There are three benefits I experienced:
First, this also allowed us to ask questions we may not have asked in person because you know that on chat you won't necessarily interrupt the speaker, so students can help students. (e.g. Student A: "which country did Professor M just mention?" Student B: "It was Egypt." T.A.: "Here's a link to the article about Egypt that Professor M was quoting from if you want to read the whole thing.")
Second, I found that this way you can also handle MANY more questions than can be managed in-person, meaning more people are given the chance to ask questions in real time. Students could answer other students' questions, which got more people engaged than having the professor answer each question.
Third, for really important questions, the professor would stop and address them, or tailor the presentation based on discussion within the chat. This allowed the professor to know how well the students were understanding the information, what the students found interesting, and even if their microphone was loud enough. :)