Shared memory space is one thing. A process has isolated memory. An error in a spawned thread could mess up shared memory, crashing multiple threads. An error i

Shared memory space is one thing. A process has isolated memory. An error in a spawned thread could mess up shared memory, crashing multiple threads. An error in a process can't mess with the memory space of other processes. Since there's no shared memory, performance characteristics change. Garbage collection is done per process and so "stop the world" doesn't happen, meaning each individual "web server" isn't at the mercy of other requests for performance. It's lots of little things like that.