Creating a connected experience has always been a goal for SimCity, and this design decision has driven our development process for the game. This is easily the most ambitious game in the franchise and we’ve taken great care to make sure that every line of code embodies the spirit of the series. To do this, we knew we had to make sure we put our heart and souls into the simulation and the team created the most powerful simulation engine in its history, the GlassBox Engine. GlassBox is the engine that drives the entire game — the buildings, the economics, trading, and also the overall simulation that can track data for up to 100,000 individual Sims inside each city. There is a massive amount of computing that goes into all of this, and GlassBox works by attributing portions of the computing to EA servers (the cloud) and some on the player’s local computer.
These guys here Interessanter DRM-Ansatz: Berechnung von Spieleteilen in der Cloud argue (in German) that the only reason to offload some calculations to the cloud is DRM, and I don’t think they’re all that wrong.