Little is documented about where the xover clip first came from, so its life online is defined mostly by how editors reuse it. The name is shorthand for crossover, and that is exactly how it is treated in short form video, as a quick marker between two ideas, characters, or scenes. Because it is only a few seconds long, it gets saved to soundboards, dropped into meme edits, and reused for transitions where a longer effect would slow the pace down and lose the joke.