IIFE (Immediately Invoked Function Expression)

What's the primary difference between an IIFE and a regular function that you call immediately after defining it?
// Example 1:
function regularFunc() {
  var x = 10;
  return x * 2;
}
var result1 = regularFunc();

// Example 2:
var result2 = (function() {
  var x = 10;
  return x * 2;
})();
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