First: In the vast majority of stories featuring the "weird", "ghostly", etc., that I have read, where there is an introduction, the editor takes some time to explain their own take on what constitutes a story in this vein. That explanation can itself sometimes be a bit obscure (e.g., Robert Aickman now and again), but it does help to give the reader some background. It is, if you will, as if the editor sets out his thesis of the weird tale, and then provides supporting examples of that theory. (Which is why even such a tale as Shirley Jackson's "One Ordinary Day, with Peanuts", can be made to fit rather well in a selection of fantasy tales, for instance.)
Second: I think Extollager has a good point with HPL and fate. Lovecraft was a mechanistic materialist, who was convinced of determinism; the complexities of the mechanism involved (the universe) is what makes it appear to we limited human beings that there is an element of "free will", but in reality everything which happens is the result of everything which has gone before -- and I mean everything -- hence far beyond the scope of our abilities to even perceive, let alone being able to calculate the influence resulting from it all. This, too, is why such a thing as chaos is perceived as such an intense horror: it is a violation of all the laws of nature and introduces truly random chance, making any sort of consistency in the universe untenable and open to dissolution at any point, present, future, or (to make it truly terrifying) even past... this last making even our memories and experiences, both individual and collective, invalid as criteria for building expectations. (Which, really, is what lies at the core of that very odd little story "Watch the Whiskers Sprout", by D. F. Lewis, in Cthulhu's Heirs.)
Third: The Dangerous Visions books are fine examples of an aspect of Ellison's approach to writing (and editing) which I don't recall seeing anyone mention: "unveiling the mystery". Throughout much of his career, Ellison has made it a point to show to both readers and even the non-readers on the street (via such things as his writing stories in shop windows and the like) that writing is a job of work, not some mysterious, esoteric practice resulting from being "touched by the gods". Not that he doesn't think there's something special about writers or writing, or any genuinely creative sort of work; but that he intends to aid in people seeing that writers work at what they do, and work hard at it. "It doesn't just come" (to quote Neil Simon's Felix Ungar). There is labor in creating these things, and there are practical considerations here as much as in building a house or managing a factory. Hence he often draws attention to these aspects of the craft, including (in his introduction to No Doors, No Windows) filling in the uninitiated on word counts and using the various stories in that collection as examples by giving the count for each, etc.) This is even more true, perhaps, of Medea: Harlan's World, which includes a massive amount of prefatory matter including a transcript of portions of the conference with the various writers involved showing how they (collectively) created a world, with considerations of its various physical, biological, and philosophical factors....