Queueing up IS contributing something back - it's a multiplayer game, so every person playing is helping keep queue times low. It also gives Riot a MAU and DAU

Queueing up IS contributing something back - it's a multiplayer game, so every person playing is helping keep queue times low. It also gives Riot a MAU and DAU which they probably track internally to determine how successful a game is (and thus how much to invest in more content for that game).