The Chosen One is a trope that predates the genre of fantasy and even literature itself. It certainly pops up in just about every religious scripture, Arthurian legend, most mythologies, and seems to permeate our modern day media, from fantasy books to anime, video games, and popular TV shows. You could even say that this trope occurs in the real world, when we hold up some politician or leader as the one that’s going to solve all our problems and save the world. Chosen Ones can be chosen by another person or entity, their lineage, a prophecy, some particular trait they possess (i.e. being “pure of heart”), or some action that they themselves take.
It’s become such a staple of the fantasy genre in particular that nearly all fantasy stories incorporate or invoke the trope in some manner. It can also be pretty lazy storytelling. It can override the hero’s agency. The plot of a Chosen One story tends to bend to this trope, along with just about every other character in the story. So what if this random person doesn’t seem like the best choice to defeat the forces of darkness? He’s the chosen one! The hero’s connection to the main conflict in the story is manufactured. It’s not personal, it’s just dictated by the nature of the trope. It’s an excuse to involve an everyman type of character in a huge, world-spanning conflict that they otherwise would have no connection to. But that is also part of the trope’s strength—that it makes a hero out of a character the audience can fully relate to, be it a high school cheerleader in Buffy the Vampire Slayer or a young son of a slave in The Phantom Menace.