Introductions

What is the purpose of an Introduction to a work of fiction?

I'm sure every editor has asked exactly that question before writing an intro (or before commissioning someone else to do so), but there certainly doesn't seem to have been an agreement anywhere. So it seems reasonable enough to ask it again. An introducing of something presumes -- doesn't it? -- that the thing being introduced, and the thing being introduced to have not met previously. Otherwise they'd be called "Reintroductions", or "Reminders". Reading decent editions of texts, though, often means being faced with (or outfaced by) a 'scholarly' introduction complete with obscure references to little things within the text, and a bamboozling array of facts (be they critical, historical, political, chronological or anything elseical). These very clever, very carefully researched critical essays are a great resource, but I don't think they serve well as introductions. They often don't make any sense whatsoever to readers coming to the text for the first time; which is surely self-defeating on the part of the introduction?

It very much depends on who is writing it. In the vast number of publications of the two editions I most frequently use (Oxford World's Classics, and Penguin) there is great variation in the apparent intentions, and end results, of the introductions. Some seem barely more than plot summaries (this is sometimes useful, because I am Not Good at remembering names of characters so it's a quick reminder; on the other hand, it doesn't tell you more than the text proper could); some are academic hard-ons (probably great fun for the writer, but a little uncomfortable for the audience); some get the balance better and provide information giving insight, but not so much that the all-important text itself is obscured behind academia so dense it causes all but the most trained eye to glaze over.

It must be a difficult balance to strike.
As I see it, an introduction should be like good gossip: more information than the original source can (or wants to) give you, but not so much sordid detail that you feel as if you're bitching unnecessarily.