Well, for me, it's just the need for the author to elaborate on things some readers might've missed. Some will say that "it's the author's job to make his/her works understandable to his/her readers", but my experience tells me the otherwise. A good, silent majority may get what you write, but the noisy 'minority' (like nitpickers) would always find something to complain.
Now it's fine to ignore them, if they won't do much damage, or you're just that chad not to care. However, not everyone is like you. And most of the time, these nitpickers would do everything to spite the author, or call attention to their whining. And the reviews they leave misunderstanding the author's vision for the work often drives away potential readers, further discouraging the writer.
Well, I may be saying my personal bias (based on experience and observation), so take this with a grain of salt. But I really do think the ability to answer to the reviews is a nice feature. Everything tends to get abused anyway: readers on the reviews coz they know the author can't refute their 'points', and authors in the comments section of their stories, coz it's their work, and they control what feedback should appear.
(Btw, I'm not promoting WebNovel, but authors can answer reviews there. Hence, in my experience, there are only a few cases where I was given 'nitpicking' reviews/feedback, thus my readers there is able to give me useful criticisms instead.)
