Sure, there is no complexity to speak of when your threads do not need to cooperate nor share any data. But then someone has a bright idea that e.g. a global c

Sure, there is no complexity to speak of when your threads do not need to cooperate nor share any data. But then someone has a bright idea that e.g. a global cache to share some state between all threads would be a good optimization, or that threads need to be able to store some global settings, etc. and complexity related to threads start to creep in to your application.