A counter example is Sivir's ultimate.
It's not NEEDED but if that is the case then so are many other aspects/abilities of other champions.
In Aatrox's lore the Protectorates were severely outnumbered and I believed had been augmented by Aatrox's presence, somehow, though powerfully.
Soon, the warriors found themselves imbued with a potent thirst for battle. They followed Aatrox into the fray, each fighting with the furious strength of ten...
I don't think it's too far-fetched to place an area of effect ally buff ability or effect on Aatrox. Conversely Sivir's lore doesn't have any scenarios as powerful as the one I just quoted and she has an area of effect ally buff ultimate.
Given all this, I don't think the Riot champion creation or Riot's champion rework team follows the translation from lore to gameplay as strict as I think they should. Sion doesn't need to have CC immunity charge ability as much as Aatrox would have an area ally buff.
There's also the gameplay philosophy with Garen that Riot tries to keep him as one of the LESS complex characters in the game since he is meant to be a starter character. For players who are new and are more focused on their own performance - therefore an area ally buff effect wouldn't be such a priority on Garen.
It's been 3 years since Riot has reworked or have taken an extensively rethink of the idea of Sivir, so their gameplay philosophy since then may have changed.