Why do most Javascript libraries append the not operator to their IIFEs?

Question

I've noticed that most/very close to all Javascript libraries (jQuery, require, moment and loads more) append their IIFE with the not operator (!). Could someone explain why this is done, and the advantages of using the not operator with IIFEs?

Here's an example of what I'm talking about.

//jquery
! function(e, t) {
  //requirejs
  var requirejs, require, define;
  ! function
  //moment
  ! function(e, t) {