the Mythic Set items with the Mythical Legend affix are a theme item. the Mythic Set helps to find Myth Stones, which are used to make Mythic affixes that turn an item into a Mythic item. since some of the Mythic affixes do damage (MH/OH Mythic, for example), the Mythic Legend affix allows the user to increase the damage of those Mythic affixes that do damage.
as far as it being a staff, from a Dev's point of view, it might have been for game balance. for the player, staffs can hold a lot of magical power compared to gauntlets, wands, or swords. at least in many fantasy stories, that is how it seems.