Some time ago I gave my brother a copy of Butcher's Crossing by John Williams, which New York Review Books had brought back into print. He remarked that enjoyed the novel, but found the introduction obnoxious. I replied that I had forgotten that the novel had an introduction, and that when I read the novel, with the introduction right there, it had not occurred to me to read it.
The copy I now have of Dawn Powell's The Locusts Have No King has a set of questions for book club discussion. Such questions strike me as implying that we in book clubs cannot come up with our own, and have to be primed like high school students. Frankly, I'd rather the publishers sank the money into a decent errata sheet, for the text here and there has distracting errors--"bridge" for "bride", "refuse" for "refuge", etc.
Certainly introductions can be useful, particularly in giving one some context: who was this author; when did he live; what else did she write? And I have read more discursive introductions that I thought just and informative, for example Louis Auchinchloss on The Bostonians and R.W.B. Lewis on The Europeans. Generally, though, I'd rather open to page 1 and start reading.
Questions I think excellent for technical matter. It would be careless to read a mathematical textbook and not work and check all the problems one can. But for the novels I'd rather find my own questions.