Maybe not the case for SimCity, but for Diablo 3 a lot of game calculations are done server side to prevent hacks in the item drop rate and item duplication. Because Diablo 3 has a real money economy, it's crucial to ensure that the items in the game are authentic and not acquired through mods/trainers/cheat programs.

Anyway, offloading processing to the server does have its benefit and uses, just maybe not the case here with SimCity (though I'm not sure about this having no experience with the series)