Prologues, in general
The first avoids tedious descriptions of how someone got into their car, drove from A to B, walked up the garden path, knocked on the door … and cuts straight to the point where they accuse the perpetrator of the crime. Meanwhile, the less of this detail the reader gets, the more their imagination fills in the blanks that we all know happened, and the more the novel becomes uniquely theirs inside their own head.
And prologues are chapter-length versions of exactly the same habit.
Well, not always. For a start, they never did Chaucer any harm, and I will admit to having used them myself. A prologue works if the characters already know it but the reader doesn’t and needs to: hence, the story is kickstarted. My novel His Majesty's Starship kicks off with a press release, which seemed the quickest way of setting the scene. But, emboldened by this, my next novel Time’s Chariot prologued with half a page of the last thoughts of a murder victim, intended to convey to the reader that this wasn’t just another murder story. To be frank, I now wish I hadn’t. Everything the reader needed to know came out later in the novel; that prologue was just words they could have skipped, and no novel should have those in them.
And I wrote Time’s Chariot before I read the novel that crystallised my thoughts on prologues: a murder mystery story. The abduction of one of the victims was witnessed and it soon became very clear to the reader that there were only a couple of people in the novel who could possibly fit the description. One of them was quite obviously the nameless gent we had met in the prologue.
Now, without the prologue, we might think “that looks like X … but he’s obviously one of the good guys and I can't think why he would do something like that, so it can't be.” Meanwhile X’s history could be revealed bit by bit and the reader would be caught up in the excitement of discovery. (Wendy H. Jones does this extremely well in her novel Killer’s Countdown, where tantalising glimpses of the killer’s past are eked out to us throughout the story - always enough to leave us wanting more, never enough to spoil things. Instead of standing outside the main story, that information becomes part of the story structure.)
But not in this case. Thanks to the prologue we could immediately guess 95% of what had happened with perfect accuracy, and see exactly why X would do that. Hence the rest of the novel – more than half of it – became a frustrating exercise of watching the hero be immensely thick until even he couldn’t avoid working it out.
Ultimately, I would say a prologue is like scaffolding. It’s unsightly; the support it offers should be intrinsic to the structure anyway; and if you can kick it away without the building falling over - why, then, it’s not needed, is it?