I'm a believer in (what I'm beginning to vaguely coalesce into a rough sort of theory I've decided to call) primacy.
Fictional stories and characters, the ones that people really fall in love with, quickly proliferate... through stories picked up by people other than the original creators, through remakes and sequels and spin-offs, or just through sheer regurgitation i.e endless issues of a comic that goes on forever and ever (DC Comics characters have been going for 75 years... that's a lot of continuity!). They become part of our mythology, but 'multiple' mythologies seem to be a part of the deal. (how many different interpretations of Sherlock Holmes has there been just in the medium of film, for instance?)
That's why primacy is so valuable. 'Primacy' here isn't a value judgement. I'm not saying (necessarily) that the 'first' version of any given modern mythology is the 'best' and should be considered higher or above any subsequent versions. But as the first version, they serve as anchors, blueprints, test cases (to mix my metaphors)... you can read/watch them to get an idea of what this character, this setting, this genre started out as being.
It's why I go back to A Study In Scarlet and The Sign Of The Four so often (for Sherlock Holmes), or the very first Hartnell stories (for Dr Who). In theory, everything you love about what came after should, even if in some embryonic form, be present in the original. Read back the very first Batman vs Joker story as created by Bob Kane and Bill Finger and then see how even the ultra-modern The Dark Knight movie has stemmed from it.
And of course, primacy has an advantage in that everyone can access it. Taking Batman again, there are now, at the moment of writing, so many different titles, ongoing continuities, universes (something called Earth One, I think... New 52... god knows what else, it's probably all changed again actually since the last time I picked up a new Batman comic a couple of years ago!). Somebody taking the plunge into all of these might have a different take from that of, say, a lapsed reader, or someone who only sticks with 'one' title (say, Detective Comics). But everyone can go back to those early stories... even if only as a reference point.
Not that any of this actually matters of course, but there is just So Much Stuff out there that I like to have the illusion of a starting point at least. There will never be an endpoint of course... and sometimes that's oddly frightening! 'Primacy' may be just as illusory a definition for a text as anything else but it has a weirdly comfortable feel to it.