I think there are two different kinds of prologues: story-prologues and world-prologues (with, of course, prologues that exist somewhere in the middle, but as types I think--particularly in the fantasy world--they can be boiled down to these two).
The story-prologue is told as story (showing more than telling; more exposition, etc) and tends to be set further in the past than the main story--possibly something tangential to the immediate action of the main story, but hugely important for where the main story is going (possibly post first-book). A world-prologue is more like the Prologue to The Lord of the Rings: really not necessary for the plot at all, but an introduction to the world.
When I was writing my fantasy novel, I initially had a story-prologue masquerading as a world-prologue, but as world-building wasn't really a major focus of the story, the separate prologue ended up being rather redundant, and I folded it over in the later drafts into the main course of the narrative, and it has rather coloured my thinking on them since as generally unnecessary--and even reading the LotR, I don't think I've generally ever started with it; I've started with Chapter 1.
Story-prologues, on the other hand, can be stylistically more useful as a foreshadowing device.
A perfect prologue would balance both varieties: introducing a story as story while also unfolding the world for the reader... but I'm really sure how such a Prologue would differ from Chapter 1. In practice, I think writers (and I include myself) like to use prologues as a crutch to get around a more artful introduction of characters and actions, but I don't know that it need be so.