What is the actual advantage of using let/block scoping?

I can't seem to see the actual advantage of using let. Sure, if I have a for loop I would use let i = 0 instead of var i = 0 and i would now belong to the scope of the for loop block. But is that really advantageous? I mean I can use var and have the variable have function scope access but just not use it where I don't need to? What is the advantage to block scoping ?

I understand the difference between var and let, I want a more coherent understanding of let's particular advantages.

Solution

You already mentioned one advantage: Block scope in loops. This avoids issues with defining functions inside a loop, which I'd argue is one of the biggest pitfalls for people new to JavaScript:

// With var
var elements = document.querySelectorAll('.var');
for (var i = 0; i < elements.length; i++) {
  elements[i].onclick = function() {
    console.log("I'm element #" + (i + 1) + ".");
  };
}

// With let
elements = document.querySelectorAll('.let');
for (let i = 0; i < elements.length; i++) {
  elements[i].onclick = function() {
    console.log("I'm element #" + (i + 1) + ".");
  };
}

<h3>With <code>var</code></h3>
<div class="var">Click me!</div>
<div class="var">Click me!</div>

<h3>With <code>let</code></h3>
<div class="let">Click me!</div>
<div class="let">Click me!</div>

Another advantage is that let variables aren't initialized by default. This makes it easier to catch mistakes when trying to access a variable with the same name in a higher scope:

// With `var`, we get `undefined` which can be tricky to debug in more
// complex code.
var foo = 42;

function bar() {
  console.log(foo); // undefined
  var foo = 21;
}
bar();

// With `let`, we get a ReferenceError, which makes it clear that we made
// a mistake here.
var foo = 42;

function bar() {
  console.log(foo); // ReferenceError
  let foo = 21;
}
bar();

All in all this aligns JavaScript more with how other languages work.