Christopher Tolkien's analysis indicates a very fluid development within the various relevant texts. Material would be yanked out of a chapter where it was presumably disrupting the flow and moved to the Prologue. An inspiration in the drafting of the Prologue would lead to alterations in the story proper. An idea that popped up in the Appendices would then propagate back through the story and the Prologue. And so on. Lots of revising and rewriting and conforming one new draft to another. Typical Tolkien.
Looking at the Prologue from a more "meta" point of view, it's a bold and fascinating authorial choice. Part of Middle-earth's enduring appeal lies in its verisimilitude -- its quality of seeming to be real or true. I think it's interesting that Tolkien foresaw the usefulness of a Prologue to set the stage from very early on.
This lengthy chunk of exposition has the effect of establishing a tone of historical authenticity. It's a way of telling you, "This all really happened." The flavor of it matches the tone of prologues found in real history books -- authoritative, meticulous, able to take a somewhat detached, amused view of its subject.
In a very economical way, Tolkien is able to suggest a broad and deep history of Middle-earth, of which the following tale will only examine a relatively small -- though significant -- part. It's a bold move which risks boring the reader before the story has properly begun. Certainly in today's publishing world, where the mantra is "Show Don't Tell", Tolkien would be obliged to battle tooth and nail to open his tale in this way.
P.S. -- Canadians fought in WWII?
P.P.S. -- Cross-posting with
Carnimírië. Perhaps my musings have somewhat addressed your question.