As we have said before, skill and ranking are never measured directly; instead, they are inferred from the observed performance of a player in previous matches. The idea of building an estimator based on pairwise wins and losses is quite old, but the Elo System represents the most simple and popular implementation.

A Ranking Systems is a core element of every competitive game. Every game in which a player plays against another player need to have some way to rank them and compare the relative skill of each other. It happens for video games, but also sport (e.g., the FIFA ranking for football) or other games (e.g., FIDE Chess Ranking). Comparing stuff is in the human nature.