I haven't done a Q&A because my series' are not popular enough to warrant one. Yet. Someday...
Anyway, I think it's unfair to say that a Q&A is unnecessary because any questions should have already been answered in the main story. In the first place, describing every little irrelevant detail in the main story can cause clutter. In the second place, a Q&A gives you the opportunity to engage with your readers and, if you've done it right, those readers have become attached to your story and would like to know more.
For example, a question I see a lot in Q&A's is "What Hogwarts house would your characters be in?"
Other common ones are "What is this character's favourite song?", "What does this character like to eat?", and so on. These might seem like silly questions, but they help readers bond more with your characters.
Obviously if you're writing a horror story and introduce a new character you're not going to say, in your main story, "Robert appeared in the hallway. He had thick black hair, a scar across his right eye, and a menacing smile. Oh, and he's totally a Slytherin. His favourite song is Dance, Dance, Dance by the Steve Miller Band and he loves the onion rings at A&W."
Q&A gives readers a chance to find out details about your characters, your story, and even you that would not otherwise be included in your work.