Well, if you say, in math, that function f() is defined by equation f(x) = 2*x + 1 + sin(x) what would you call the "x"? A variable. And does it change within evaluation of the 2*x + 1 + sin(x)? The sentence that variables can't change value is not really correct actually. Next time you evaluate a function the x defined as its parameter or as an internal variable may have a totally different value. It's just that instead of "hey, I want to have a blackboard on which I'll write numbers and I'll call it X. Write 5 there now! OK and now clean it and write 10 there!" you just say "the value of something I wish to call X depends on the parameters of the function like this". And it's just the habits from imperative language that force you to look for ways to overwrite the X later instead of using a different name for the result of a different computation.

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