It doesn't have the same semantics, because even after introducing the new variable, it is still immutable. So you cannot accidentally modify the new variable. e.g.,

  let x = 1;
  let x = x + 1;
  x = 99; // Error: you can't accidentally modify the immutable variable `x`

  let mut x = 1;
  x = x + 1;
  x = 99; // Ok

You can even turn back a mutable varaible into an immutable variable, like this:

  let mut x = 1;
  x = x + 1;
  let x = x;
  x = 99; // Error: `x` became immutable

This can be useful to "pin" the variable after a large number of calculations done to it.