If variables don't mutate, all it means is that the memory location that stores the value of that variable is guaranteed to never change. Thus, if you reassign that same-named variable, it actually points to a new memory location with the new value. That is what "rebinding" is vs. "mutating." The advantage of this is concurrency, etc. etc. The disadvantage is you have to have good GC since you'll be doing a lot more garbage collection. It may LOOK like it's changing a variable, but it's not- the variable merely points to a new memory location.

Other languages, like Ruby, do not work this way. If you reassign an instance variable, the memory that is storing that value actually does change.

Here, compare the same sequence of operations in Ruby and Elixir, first Ruby:

    bash>> irb
    >> a = {a: 5}
    => {:a=>5}
    >> b = a
    => {:a=>5}
    >> a[:a] = 10
    => 10
    >> b
    => {:a=>10}

and now Elixir:

    bash>> iex
    Erlang/OTP 18 [erts-7.1] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

    Interactive Elixir (1.1.0) - press Ctrl+C to exit (type h() ENTER for help)
    iex(1)> a = %{a: 5}
    %{a: 5}
    iex(2)> b = a
    %{a: 5}
    iex(3)> a = %{a | a: 10} # updates a mapping
    %{a: 10}
    iex(4)> b
    %{a: 5}

Notice how b's value changed in Ruby, but not in Elixir. In both cases, the hash was updated.