When you have over 12 million people playing your game every single day, a lot of data is getting moved around. How much? In League of Legends' case, over 4 terabytes worth.
In a 24-hour period, the game's servers have to deal with "500 GB of structured data and over four TB of operational logs", a staggering amount of 1s and 0s that earns it all the technical term "big data".
The specifics on how it's all managed are, to be honest, a little lost on me, but if you're more well-versed you should check out this interview on Slashdot, which talks stuff like how where there was once a single data analyst on staff, Riot now employs two dozen people just to look after moving all its data around.
For Riot Games, Big Data Is Serious Business [Slashdot]