I think the motivation for actors is that it's an abstraction which lets you write concurrent code with reasonably strong safety guarantees without adding as mu

I think the motivation for actors is that it's an abstraction which lets you write concurrent code with reasonably strong safety guarantees without adding as much cognitive overhead as a strict system like Rust. GCD is a really nice abstraction for async, but it doesn't protect you from shooting yourself in the foot if you're not careful about what thread is doing what. That said, I'm really not sold on actors. I think CS as a whole hasn't really landed on the right abstraction for concurrency yet, actors seem a bit experimental, and GCD is plenty good enough in most cases.