You can certainly achieve the same low coupling with anything. But .... Messages tend to be lightweight. Often a symbol/atom "command", and a few scalar values

You can certainly achieve the same low coupling with anything. But .... Messages tend to be lightweight. Often a symbol/atom "command", and a few scalar values. Like command line arguments. If the process needs more data, it sends its own message to the owner of said data. With conventional functions and OO, you're often dealing with complex objects (with references to references ...) which often leads to breaking the Law of Demeter. Message passing also gets your asynchronicity. You're also decoupled from specific runtime implementation (the receiving can be on a different machine, the semantics are the same). You can achieve the same without message passing, but it's certainly more baked-into the runtime/tooling/language. Joe Armstrong's popular quote on this: "The problem with object-oriented languages is they’ve got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle"