[zelphirkalt]

Can I make a stupid example? Lets assume you got some building with people inside. One person exits the building. mybuilding.leave(person) or so. What does .leave(...) do? Ah, it needs to reduce the counter of people in the building by 1. Oh and by convention this means, that the person is checking out, so lets update their schedule to account for that. Ah they are also currently "out of office". Ah and the person at the door, who keeps watch has now seen another person leaving. Maybe the person went through a scanner. The scanner statistics need to be updated.

And so on and on. All kinds of objects need updates for their states. You need to direct all this stuff, without forgetting things. Otherwise the state of your objects will become inconsistent and at some point it will show as some kind of bug. At that point you might find yourself debugging the whole thing, to find out when the inconsistency starts or what causes it.

No need for multiple threads or any of the kind. Complications from splitting the state up and parking it in various objects and having to tell the computer how to update these objects. Instead of looking at 1 function at a time, which only works on its inputs and gives an output, you have to remember to call all the appropriate methods across some reasonable part of your entity landscape.