I know, but very rarely, especially in smaller world's, a world may end up with absolutely none of a rare item. If that item is required or highly recommended for progressing, the player is screwed.
Says someone who doesn't give a crap about achievements. You get Xbox achievements for half Hallowing and/or half corrupting your world, obtaining all pets (see my a world may generate none of a rare object quote. Also, a new player is more likely to learn about the Bone Key post-Skeletron.), removing thousands of blocks (someone might need a digging world if they don't have good pick.). as well, EOW and BOC have separate in game achievements, meaning to get all achievements you have to make multiple world's.