Javascript why wrap a variable or constructor in an IIFE?

There's no point for the specific construct that you show here. The reason to use an IIFE in this type of construct is when you have static data that you need to declare, want to be available to your object, but don't want it to be publicly accessible or interfere with the global namespace or be instance data.

Since the code you show doesn't show any of those, it isn't really offering any benefit as you've shown. But, if there were some other variables declared outside the object, but inside the IIFE, then the IIFE would protect and enclose them and isolate them from the outside world.

For example, if you had this:

Visualizer = (function() {
  var counter = 0;
  function Visualizer() {
    counter++;
    ...
  }
  Visualizer.prototype.getCount = function () { return counter; }
  ...
  return Visualizer;
})();

var viz = new Visualizer();

Then, the IIFE would be enclosing a variable counter that would be available to all methods of all instances of Visualizer, but isolated from the outside world and the IIFE would be offering some potential benefit.