Its less because its a legacy thing, and more because its just a massive trope for spellcasters to need to chant in arcane languages or make hand motions.This is one of those Legacy things that I think WotC keeps in the game for like the 6 people across the globe who actually care about components.
And, yes, its part of the whole gagged, hands bound wizard schtick.
Look, it does rarely come up in play, but that doesn't mean that its not a necessary part of a standard fantasy story.