In HG/SS Arceus unlocks Shinjo Ruins (may be done 2 times, one with event Arceus, one with Azure Flute Arceus) where you can get Dialga/Palkia/Giratina at lv.1.
Shiny beasts (Entei/Raikou/Suicune) from 12th Movie Giveaway unlocks some Zoroark event in B/W.
And, event-key items are really key, first you get them (eg. Oak's Letter, Member Card or Azure Flute) then catch legendaries (Shaymin, Darkrai or Arceus). The same is true for older games, like tickets, maps or keys.