Those titles are a relic of when League seemed like just DotA with its own client instead of in WC3. Now that they've made over 100 of them, it just seems like Riot is stuck giving all of the champs a title, even if it doesn't make much sense to name them that way.
In DotA 2, it's true, there is almost no uniformity to this, with people calling them interchangeably and never giving much reason why they go by which name. He's Banehallow the Lycanthropy, but the accepted name is just Lycan now; or Rikimaru the Stealth Assassin, but he goes by Riki.
League was modeled after DotA, even down to the way they name their champions. But, like most things, League ended up doing it better and at least has uniformity with the names to prevent ambiguity, even if the titles still exist.