I think it likely that the story evolved memetically, rather than being "for" anything in particular. It might have started with something as simple as Newton mentioning a falling fruit by way of illustration during a lecture, but as people pass the story around embellishments that resonate stack up. Apples, as you note, have rich cultural connotations involving intelligence in general and instantaneous revelation in particular (interestingly, Genesis does not actually mention apples, referring only to "fruit", which would seem to suggest that apples get their symbolism from somewhere else. Possibly their brain-like cross section?). The more recent retellings have added the embellishment that Newton was not just inspired by a falling apple but literally struck by one, the apple serving as a physical manifestation of an idea.

So I think the guiding principle is not so much "how can we poetically represent Newton" as "what do humans think is cool".