When you have a large amount of data that needs to be sequentially iterated through by multiple threads — e.g. in games, where the UI thread and the physics thr

When you have a large amount of data that needs to be sequentially iterated through by multiple threads — e.g. in games, where the UI thread and the physics thread are often entirely separate but read off a shared world state — message passing falls over. The copies are just too expensive. Message passing is great for things that use small amounts of data but lots of CPU, though!