I assume that you know what the functional programming is.
A great question (and maybe your first question) is:
“Why do we need functional programming?”
“Why would an architect be concerned with the mutability of variables?”
The answer is absurdly simple: All race conditions, deadlock conditions, and concurrent update problems are due to mutable variables. You cannot have a race condition or a concurrent update problem if no variable is ever updated.
You cannot have deadlocks without mutable locks.
In other words, all the problems that we face in concurrent applications - all the problems we face in applications that require multiple threads, and multiple processors - cannot happen if there are no mutable variables.
Although there are lots of things to discuss about this statement, but this is the main point.
You can learn more by reading that book from Uncle Bob.