[logicchains]

The advantage is you can have many more green threads than you could have OS threads (hundreds of thousands vs thousands), due to green threads being more lightweight. This allows a programming model based on message passing between green threads, which many people consider nicer to reason about. E.g. for a web server you could have a goroutine/Erlang process for each client with 50k clients and still have excellent performance, whereas if you had 50k OS threads you'd likely suffer performance issues and use a heap more ram.

[EdHominem]

And this is a huge benefit in the ease of coding.

If you only have two or four or sixteen threads you write overtly threaded code. But when you have millions you don't. Your program looks single-threaded and yet operates better and more safely.