I ready about this the other day and I'm very excited.
League has had custom games forever, but it was never a sandbox. You still had to play against bots or other players, level up as normal, etc. With this new mode, it will allow players to quickly try out different item builds, skill order leveling up, etc. It should be pretty awesome. You could always do this before, but you had to play a game, now you can just set the parameters and have at it. Very cool Riot, very cool.