"et in medias res (...) auditorem rapit"
and throw your audience in the thick of things.
For were the story to have started so: "Once upon a tyme, when hadde the fleah 200-pound horseshoes put on and jumped high up to the welkin, but the welkin was cloudy and the fleah wended to the ground, smitten by lyghtnyng" so many things would have been needful of an explanation such as might forever stall the story in a manner most unpolite and unprofitable.
As I was saying