IIFE vs. plain function

gistfile1.js

function someFunction() {
 
  
  // both methods do exactly the same thing. where is the difference then?
  
  var reader = new FileReader();
  var img = document.createElement('img');
  
  // method A
  reader.onload = (function(aImg) {
    return function(e) {
      aImg.src = e.target.result;
    };
  })(img);
  
  // method B
  reader.onload = function(e) {
    img.src = e.target.result;
  }
  
  reader.readAsDataURL(file);
}