At some point, someone (perhaps Dave Wesely, perhaps Gary Gygax) found a twenty-sided dice supplier in the US: Creative Publications of California. However they sold their dice in packs of five, including a d4, a d6, a d8, a d12, and a d20. Notably missing was the d10, which didn't appear until Gen Con XIII (1980). There was one problem with Creative Publications' dice: D&D only needed the d6 and the d20. TSR decided to use the full sets to avoid having to sort out the other polyhedrons when they sold them. As a result, OD&D added some rules for using the other dice — particularly in the number appearing charts for monsters. Nonetheless, they were pretty scant.
This makes so much sense: OD&D definitely has the flavor of a game that is barely a few inches away from just needing d6's and d20's. And frankly: I want that game. So incredibly elegant! (Recall: My monster hit dice and damage are still all d6's; to date players have never noticed any difference, and its so much easier to manage as the DM).