All languages will have side effects. They will have some form of mutability. The whole goal is to try to decrease this mutability as much as possibl… - Francesco Cesarini

When you are writing a massively complex highly distributed system, you need more just than a language, Erlang is just a programming language. You need architectural patterns, you need to start thinking in term of how you distribute your system.

There is two ways basically to deal with concurrency. Mutable state, you must have shared memory, you must have threads, you must have mutexes when you access critical sections. If you are dealing with immutable state, what you do is processes will not share memory. The only way for them to share data is through a message passing that needs to copy the data from one process to another. They'll each have their own copy. So, two completely different paradigms, two completely different results.

By learning them, and understanding their paradigms, and by programming in a functional way, you will end up being much more productive and you're becoming much better programmers.